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perf_event_open(2) System Calls Manualperf_event_open(2)perf_event_open - set up performance monitoring
Standard C library (libc,-lc)
#include <linux/perf_event.h>/* Definition ofPERF_*constants */#include <linux/hw_breakpoint.h>/* Definition ofHW_*constants */#include <sys/syscall.h>/* Definition ofSYS_*constants */#include <unistd.h>int syscall(SYS_perf_event_open, struct perf_event_attr *attr,pid_tpid, intcpu, intgroup_fd, unsigned longflags);Note: glibc provides no wrapper forperf_event_open(), necessitating the use ofsyscall(2).
Given a list of parameters,perf_event_open() returns a file descriptor, for use in subsequent system calls (read(2),mmap(2),prctl(2),fcntl(2), etc.). A call toperf_event_open() creates a file descriptor that allows measuring performance information. Each file descriptor corresponds to one event that is measured; these can be grouped together to measure multiple events simultaneously. Events can be enabled and disabled in two ways: viaioctl(2) and viaprctl(2). When an event is disabled it does not count or generate overflows but does continue to exist and maintain its count value. Events come in two flavors: counting and sampled. Acounting event is one that is used for counting the aggregate number of events that occur. In general, counting event results are gathered with aread(2) call. Asampling event periodically writes measurements to a buffer that can then be accessed viammap(2).Arguments Thepid andcpu arguments allow specifying which process and CPU to monitor:pid == 0andcpu == -1 This measures the calling process/thread on any CPU.pid == 0andcpu >= 0 This measures the calling process/thread only when running on the specified CPU.pid > 0andcpu == -1 This measures the specified process/thread on any CPU.pid > 0andcpu >= 0 This measures the specified process/thread only when running on the specified CPU.pid == -1andcpu >= 0 This measures all processes/threads on the specified CPU. This requiresCAP_PERFMON(since Linux 5.8) orCAP_SYS_ADMINcapability or a/proc/sys/kernel/perf_event_paranoid value of less than 1.pid == -1andcpu == -1 This setting is invalid and will return an error. Whenpid is greater than zero, permission to perform this system call is governed byCAP_PERFMON(since Linux 5.9) and a ptrace access modePTRACE_MODE_READ_REALCREDScheck on older Linux versions; seeptrace(2). Thegroup_fd argument allows event groups to be created. An event group has one event which is the group leader. The leader is created first, withgroup_fd = -1. The rest of the group members are created with subsequentperf_event_open() calls withgroup_fd being set to the file descriptor of the group leader. (A single event on its own is created withgroup_fd = -1 and is considered to be a group with only 1 member.) An event group is scheduled onto the CPU as a unit: it will be put onto the CPU only if all of the events in the group can be put onto the CPU. This means that the values of the member events can be meaningfully compared —added, divided (to get ratios), and so on— with each other, since they have counted events for the same set of executed instructions. Theflags argument is formed by ORing together zero or more of the following values:PERF_FLAG_FD_CLOEXEC(since Linux 3.14) This flag enables the close-on-exec flag for the created event file descriptor, so that the file descriptor is automatically closed onexecve(2). Setting the close-on- exec flags at creation time, rather than later withfcntl(2), avoids potential race conditions where the calling thread invokesperf_event_open() andfcntl(2) at the same time as another thread callsfork(2) thenexecve(2).PERF_FLAG_FD_NO_GROUP This flag tells the event to ignore thegroup_fd parameter except for the purpose of setting up output redirection using thePERF_FLAG_FD_OUTPUTflag.PERF_FLAG_FD_OUTPUT(broken since Linux 2.6.35) This flag re-routes the event's sampled output to instead be included in the mmap buffer of the event specified bygroup_fd.PERF_FLAG_PID_CGROUP(since Linux 2.6.39) This flag activates per-container system-wide monitoring. A container is an abstraction that isolates a set of resources for finer-grained control (CPUs, memory, etc.). In this mode, the event is measured only if the thread running on the monitored CPU belongs to the designated container (cgroup). The cgroup is identified by passing a file descriptor opened on its directory in the cgroupfs filesystem. For instance, if the cgroup to monitor is calledtest, then a file descriptor opened on/dev/cgroup/test (assuming cgroupfs is mounted on/dev/cgroup) must be passed as thepid parameter. cgroup monitoring is available only for system-wide events and may therefore require extra permissions. Theperf_event_attr structure provides detailed configuration information for the event being created. struct perf_event_attr { __u32 type; /* Type of event */ __u32 size; /* Size of attribute structure */ __u64 config; /* Type-specific configuration */ union { __u64 sample_period; /* Period of sampling */ __u64 sample_freq; /* Frequency of sampling */ }; __u64 sample_type; /* Specifies values included in sample */ __u64 read_format; /* Specifies values returned in read */ __u64 disabled : 1, /* off by default */ inherit : 1, /* children inherit it */ pinned : 1, /* must always be on PMU */ exclusive : 1, /* only group on PMU */ exclude_user : 1, /* don't count user */ exclude_kernel : 1, /* don't count kernel */ exclude_hv : 1, /* don't count hypervisor */ exclude_idle : 1, /* don't count when idle */ mmap : 1, /* include mmap data */ comm : 1, /* include comm data */ freq : 1, /* use freq, not period */ inherit_stat : 1, /* per task counts */ enable_on_exec : 1, /* next exec enables */ task : 1, /* trace fork/exit */ watermark : 1, /* wakeup_watermark */ precise_ip : 2, /* skid constraint */ mmap_data : 1, /* non-exec mmap data */ sample_id_all : 1, /* sample_type all events */ exclude_host : 1, /* don't count in host */ exclude_guest : 1, /* don't count in guest */ exclude_callchain_kernel : 1, /* exclude kernel callchains */ exclude_callchain_user : 1, /* exclude user callchains */ mmap2 : 1, /* include mmap with inode data */ comm_exec : 1, /* flag comm events that are due to exec */ use_clockid : 1, /* use clockid for time fields */ context_switch : 1, /* context switch data */ write_backward : 1, /* Write ring buffer from end to beginning */ namespaces : 1, /* include namespaces data */ ksymbol : 1, /* include ksymbol events */ bpf_event : 1, /* include bpf events */ aux_output : 1, /* generate AUX records instead of events */ cgroup : 1, /* include cgroup events */ text_poke : 1, /* include text poke events */ build_id : 1, /* use build id in mmap2 events */ inherit_thread : 1, /* children only inherit */ /* if cloned with CLONE_THREAD */ remove_on_exec : 1, /* event is removed from task on exec */ sigtrap : 1, /* send synchronous SIGTRAP on event */ __reserved_1 : 26; union { __u32 wakeup_events; /* wakeup every n events */ __u32 wakeup_watermark; /* bytes before wakeup */ }; __u32 bp_type; /* breakpoint type */ union { __u64 bp_addr; /* breakpoint address */ __u64 kprobe_func; /* for perf_kprobe */ __u64 uprobe_path; /* for perf_uprobe */ __u64 config1; /* extension of config */ }; union { __u64 bp_len; /* breakpoint size */ __u64 kprobe_addr; /* with kprobe_func == NULL */ __u64 probe_offset; /* for perf_[k,u]probe */ __u64 config2; /* extension of config1 */ }; __u64 branch_sample_type; /* enum perf_branch_sample_type */ __u64 sample_regs_user; /* user regs to dump on samples */ __u32 sample_stack_user; /* size of stack to dump on samples */ __s32 clockid; /* clock to use for time fields */ __u64 sample_regs_intr; /* regs to dump on samples */ __u32 aux_watermark; /* aux bytes before wakeup */ __u16 sample_max_stack; /* max frames in callchain */ __u16 __reserved_2; /* align to u64 */ __u32 aux_sample_size; /* max aux sample size */ __u32 __reserved_3; /* align to u64 */ __u64 sig_data; /* user data for sigtrap */ }; The fields of theperf_event_attr structure are described in more detail below:type This field specifies the overall event type. It has one of the following values:PERF_TYPE_HARDWARE This indicates one of the "generalized" hardware events provided by the kernel. See theconfig field definition for more details.PERF_TYPE_SOFTWARE This indicates one of the software-defined events provided by the kernel (even if no hardware support is available).PERF_TYPE_TRACEPOINT This indicates a tracepoint provided by the kernel tracepoint infrastructure.PERF_TYPE_HW_CACHE This indicates a hardware cache event. This has a special encoding, described in theconfig field definition.PERF_TYPE_RAW This indicates a "raw" implementation-specific event in theconfig field.PERF_TYPE_BREAKPOINT(since Linux 2.6.33) This indicates a hardware breakpoint as provided by the CPU. Breakpoints can be read/write accesses to an address as well as execution of an instruction address. dynamic PMU Since Linux 2.6.38,perf_event_open() can support multiple PMUs. To enable this, a value exported by the kernel can be used in thetype field to indicate which PMU to use. The value to use can be found in the sysfs filesystem: there is a subdirectory per PMU instance under/sys/bus/event_source/devices. In each subdirectory there is atype file whose content is an integer that can be used in thetype field. For instance,/sys/bus/event_source/devices/cpu/type contains the value for the core CPU PMU, which is usually 4.kprobeuprobe(both since Linux 4.17) These two dynamic PMUs create a kprobe/uprobe and attach it to the file descriptor generated by perf_event_open. The kprobe/uprobe will be destroyed on the destruction of the file descriptor. See fieldskprobe_func,uprobe_path,kprobe_addr, andprobe_offset for more details.size The size of theperf_event_attr structure for forward/backward compatibility. Set this usingsizeof(struct perf_event_attr) to allow the kernel to see the struct size at the time of compilation. The related definePERF_ATTR_SIZE_VER0is set to 64; this was the size of the first published struct.PERF_ATTR_SIZE_VER1is 72, corresponding to the addition of breakpoints in Linux 2.6.33.PERF_ATTR_SIZE_VER2is 80 corresponding to the addition of branch sampling in Linux 3.4.PERF_ATTR_SIZE_VER3is 96 corresponding to the addition ofsample_regs_user andsample_stack_user in Linux 3.7.PERF_ATTR_SIZE_VER4is 104 corresponding to the addition ofsample_regs_intr in Linux 3.19.PERF_ATTR_SIZE_VER5is 112 corresponding to the addition ofaux_watermark in Linux 4.1.config This specifies which event you want, in conjunction with thetype field. Theconfig1 andconfig2 fields are also taken into account in cases where 64 bits is not enough to fully specify the event. The encoding of these fields are event dependent. There are various ways to set theconfig field that are dependent on the value of the previously describedtype field. What follows are various possible settings forconfig separated out bytype. Iftype isPERF_TYPE_HARDWARE, we are measuring one of the generalized hardware CPU events. Not all of these are available on all platforms. Setconfig to one of the following:PERF_COUNT_HW_CPU_CYCLES Total cycles. Be wary of what happens during CPU frequency scaling.PERF_COUNT_HW_INSTRUCTIONS Retired instructions. Be careful, these can be affected by various issues, most notably hardware interrupt counts.PERF_COUNT_HW_CACHE_REFERENCES Cache accesses. Usually this indicates Last Level Cache accesses but this may vary depending on your CPU. This may include prefetches and coherency messages; again this depends on the design of your CPU.PERF_COUNT_HW_CACHE_MISSES Cache misses. Usually this indicates Last Level Cache misses; this is intended to be used in conjunction with thePERF_COUNT_HW_CACHE_REFERENCESevent to calculate cache miss rates.PERF_COUNT_HW_BRANCH_INSTRUCTIONS Retired branch instructions. Prior to Linux 2.6.35, this used the wrong event on AMD processors.PERF_COUNT_HW_BRANCH_MISSES Mispredicted branch instructions.PERF_COUNT_HW_BUS_CYCLES Bus cycles, which can be different from total cycles.PERF_COUNT_HW_STALLED_CYCLES_FRONTEND(since Linux 3.0) Stalled cycles during issue.PERF_COUNT_HW_STALLED_CYCLES_BACKEND(since Linux 3.0) Stalled cycles during retirement.PERF_COUNT_HW_REF_CPU_CYCLES(since Linux 3.3) Total cycles; not affected by CPU frequency scaling. Iftype isPERF_TYPE_SOFTWARE, we are measuring software events provided by the kernel. Setconfig to one of the following:PERF_COUNT_SW_CPU_CLOCK This reports the CPU clock, a high-resolution per-CPU timer.PERF_COUNT_SW_TASK_CLOCK This reports a clock count specific to the task that is running.PERF_COUNT_SW_PAGE_FAULTS This reports the number of page faults.PERF_COUNT_SW_CONTEXT_SWITCHES This counts context switches. Until Linux 2.6.34, these were all reported as user-space events, after that they are reported as happening in the kernel.PERF_COUNT_SW_CPU_MIGRATIONS This reports the number of times the process has migrated to a new CPU.PERF_COUNT_SW_PAGE_FAULTS_MIN This counts the number of minor page faults. These did not require disk I/O to handle.PERF_COUNT_SW_PAGE_FAULTS_MAJ This counts the number of major page faults. These required disk I/O to handle.PERF_COUNT_SW_ALIGNMENT_FAULTS(since Linux 2.6.33) This counts the number of alignment faults. These happen when unaligned memory accesses happen; the kernel can handle these but it reduces performance. This happens only on some architectures (never on x86).PERF_COUNT_SW_EMULATION_FAULTS(since Linux 2.6.33) This counts the number of emulation faults. The kernel sometimes traps on unimplemented instructions and emulates them for user space. This can negatively impact performance.PERF_COUNT_SW_DUMMY(since Linux 3.12) This is a placeholder event that counts nothing. Informational sample record types such as mmap or comm must be associated with an active event. This dummy event allows gathering such records without requiring a counting event.PERF_COUNT_SW_BPF_OUTPUT(since Linux 4.4) This is used to generate raw sample data from BPF. BPF programs can write to this event usingbpf_perf_event_outputhelper.PERF_COUNT_SW_CGROUP_SWITCHES(since Linux 5.13) This counts context switches to a task in a different cgroup. In other words, if the next task is in the same cgroup, it won't count the switch. Iftype isPERF_TYPE_TRACEPOINT, then we are measuring kernel tracepoints. The value to use inconfig can be obtained from under debugfstracing/events/*/*/id if ftrace is enabled in the kernel. Iftype isPERF_TYPE_HW_CACHE, then we are measuring a hardware CPU cache event. To calculate the appropriateconfig value, use the following equation: config = (perf_hw_cache_id) | (perf_hw_cache_op_id << 8) | (perf_hw_cache_op_result_id << 16); whereperf_hw_cache_id is one of:PERF_COUNT_HW_CACHE_L1D for measuring Level 1 Data CachePERF_COUNT_HW_CACHE_L1I for measuring Level 1 Instruction CachePERF_COUNT_HW_CACHE_LL for measuring Last-Level CachePERF_COUNT_HW_CACHE_DTLB for measuring the Data TLBPERF_COUNT_HW_CACHE_ITLB for measuring the Instruction TLBPERF_COUNT_HW_CACHE_BPU for measuring the branch prediction unitPERF_COUNT_HW_CACHE_NODE(since Linux 3.1) for measuring local memory accesses andperf_hw_cache_op_id is one of:PERF_COUNT_HW_CACHE_OP_READ for read accessesPERF_COUNT_HW_CACHE_OP_WRITE for write accessesPERF_COUNT_HW_CACHE_OP_PREFETCH for prefetch accesses andperf_hw_cache_op_result_id is one of:PERF_COUNT_HW_CACHE_RESULT_ACCESS to measure accessesPERF_COUNT_HW_CACHE_RESULT_MISS to measure misses Iftype isPERF_TYPE_RAW, then a custom "raw"config value is needed. Most CPUs support events that are not covered by the "generalized" events. These are implementation defined; see your CPU manual (for example the Intel Volume 3B documentation or the AMD BIOS and Kernel Developer Guide). The libpfm4 library can be used to translate from the name in the architectural manuals to the raw hex valueperf_event_open() expects in this field. Iftype isPERF_TYPE_BREAKPOINT, then leaveconfig set to zero. Its parameters are set in other places. Iftype iskprobeoruprobe, setretprobe (bit 0 ofconfig, see/sys/bus/event_source/devices/[k,u]probe/format/retprobe) for kretprobe/uretprobe. See fieldskprobe_func,uprobe_path,kprobe_addr, andprobe_offset for more details.kprobe_funcuprobe_pathkprobe_addrprobe_offset These fields describe the kprobe/uprobe for dynamic PMUskprobeanduprobe. Forkprobe: usekprobe_func andprobe_offset, or usekprobe_addr and leavekprobe_func as NULL. Foruprobe: useuprobe_path andprobe_offset.sample_periodsample_freq A "sampling" event is one that generates an overflow notification every N events, where N is given bysample_period. A sampling event hassample_period > 0. When an overflow occurs, requested data is recorded in the mmap buffer. Thesample_type field controls what data is recorded on each overflow.sample_freq can be used if you wish to use frequency rather than period. In this case, you set thefreq flag. The kernel will adjust the sampling period to try and achieve the desired rate. The rate of adjustment is a timer tick.sample_type The various bits in this field specify which values to include in the sample. They will be recorded in a ring- buffer, which is available to user space usingmmap(2). The order in which the values are saved in the sample are documented in the MMAP Layout subsection below; it is not theenum perf_event_sample_format order.PERF_SAMPLE_IP Records instruction pointer.PERF_SAMPLE_TID Records the process and thread IDs.PERF_SAMPLE_TIME Records a timestamp.PERF_SAMPLE_ADDR Records an address, if applicable.PERF_SAMPLE_READ Record counter values for all events in a group, not just the group leader.PERF_SAMPLE_CALLCHAIN Records the callchain (stack backtrace).PERF_SAMPLE_ID Records a unique ID for the opened event's group leader.PERF_SAMPLE_CPU Records CPU number.PERF_SAMPLE_PERIOD Records the current sampling period.PERF_SAMPLE_STREAM_ID Records a unique ID for the opened event. UnlikePERF_SAMPLE_IDthe actual ID is returned, not the group leader. This ID is the same as the one returned byPERF_FORMAT_ID.PERF_SAMPLE_RAW Records additional data, if applicable. Usually returned by tracepoint events.PERF_SAMPLE_BRANCH_STACK(since Linux 3.4) This provides a record of recent branches, as provided by CPU branch sampling hardware (such as Intel Last Branch Record). Not all hardware supports this feature. See thebranch_sample_type field for how to filter which branches are reported.PERF_SAMPLE_REGS_USER(since Linux 3.7) Records the current user-level CPU register state (the values in the process before the kernel was called).PERF_SAMPLE_STACK_USER(since Linux 3.7) Records the user level stack, allowing stack unwinding.PERF_SAMPLE_WEIGHT(since Linux 3.10) Records a hardware provided weight value that expresses how costly the sampled event was. This allows the hardware to highlight expensive events in a profile.PERF_SAMPLE_DATA_SRC(since Linux 3.10) Records the data source: where in the memory hierarchy the data associated with the sampled instruction came from. This is available only if the underlying hardware supports this feature.PERF_SAMPLE_IDENTIFIER(since Linux 3.12) Places theSAMPLE_IDvalue in a fixed position in the record, either at the beginning (for sample events) or at the end (if a non-sample event). This was necessary because a sample stream may have records from various different event sources with differentsample_type settings. Parsing the event stream properly was not possible because the format of the record was needed to findSAMPLE_ID, but the format could not be found without knowing what event the sample belonged to (causing a circular dependency). ThePERF_SAMPLE_IDENTIFIERsetting makes the event stream always parsable by puttingSAMPLE_IDin a fixed location, even though it means having duplicateSAMPLE_IDvalues in records.PERF_SAMPLE_TRANSACTION(since Linux 3.13) Records reasons for transactional memory abort events (for example, from Intel TSX transactional memory support). Theprecise_ip setting must be greater than 0 and a transactional memory abort event must be measured or no values will be recorded. Also note that some perf_event measurements, such as sampled cycle counting, may cause extraneous aborts (by causing an interrupt during a transaction).PERF_SAMPLE_REGS_INTR(since Linux 3.19) Records a subset of the current CPU register state as specified bysample_regs_intr. UnlikePERF_SAMPLE_REGS_USERthe register values will return kernel register state if the overflow happened while kernel code is running. If the CPU supports hardware sampling of register state (i.e., PEBS on Intel x86) andprecise_ip is set higher than zero then the register values returned are those captured by hardware at the time of the sampled instruction's retirement.PERF_SAMPLE_PHYS_ADDR(since Linux 4.13) Records physical address of data like inPERF_SAMPLE_ADDR.PERF_SAMPLE_CGROUP(since Linux 5.7) Records (perf_event) cgroup ID of the process. This corresponds to theid field in thePERF_RECORD_CGROUPevent.PERF_SAMPLE_DATA_PAGE_SIZE(since Linux 5.11) Records page size of data like inPERF_SAMPLE_ADDR.PERF_SAMPLE_CODE_PAGE_SIZE(since Linux 5.11) Records page size of ip like inPERF_SAMPLE_IP.PERF_SAMPLE_WEIGHT_STRUCT(since Linux 5.12) Records hardware provided weight values like inPERF_SAMPLE_WEIGHT, but it can represent multiple values in a struct. This shares the same space asPERF_SAMPLE_WEIGHT, so users can apply either of those, not both. It has the following format and the meaning of each field is dependent on the hardware implementation. union perf_sample_weight { u64 full; /* PERF_SAMPLE_WEIGHT */ struct { /* PERF_SAMPLE_WEIGHT_STRUCT */ u32 var1_dw; u16 var2_w; u16 var3_w; }; };read_format This field specifies the format of the data returned byread(2) on aperf_event_open() file descriptor.PERF_FORMAT_TOTAL_TIME_ENABLED Adds the 64-bittime_enabled field. This can be used to calculate estimated totals if the PMU is overcommitted and multiplexing is happening.PERF_FORMAT_TOTAL_TIME_RUNNING Adds the 64-bittime_running field. This can be used to calculate estimated totals if the PMU is overcommitted and multiplexing is happening.PERF_FORMAT_ID Adds a 64-bit unique value that corresponds to the event group.PERF_FORMAT_GROUP Allows all counter values in an event group to be read with one read.PERF_FORMAT_LOST (since Linux 6.0) Adds a 64-bit value that is the number of lost samples for this event. This would be only meaningful whensample_period orsample_freq is set.disabled Thedisabled bit specifies whether the counter starts out disabled or enabled. If disabled, the event can later be enabled byioctl(2),prctl(2), orenable_on_exec. When creating an event group, typically the group leader is initialized withdisabled set to 1 and any child events are initialized withdisabled set to 0. Despitedisabled being 0, the child events will not start until the group leader is enabled.inherit Theinherit bit specifies that this counter should count events of child tasks as well as the task specified. This applies only to new children, not to any existing children at the time the counter is created (nor to any new children of existing children). Inherit does not work for some combinations ofread_format values, such asPERF_FORMAT_GROUP. Additionally, using it together withcpu == -1 prevents the creation of the mmap ring-buffer used for logging asynchronous events in sampled mode.pinned Thepinned bit specifies that the counter should always be on the CPU if at all possible. It applies only to hardware counters and only to group leaders. If a pinned counter cannot be put onto the CPU (e.g., because there are not enough hardware counters or because of a conflict with some other event), then the counter goes into an 'error' state, where reads return end-of-file (i.e.,read(2) returns 0) until the counter is subsequently enabled or disabled.exclusive Theexclusive bit specifies that when this counter's group is on the CPU, it should be the only group using the CPU's counters. In the future this may allow monitoring programs to support PMU features that need to run alone so that they do not disrupt other hardware counters. Note that many unexpected situations may prevent events with theexclusive bit set from ever running. This includes any users running a system-wide measurement as well as any kernel use of the performance counters (including the commonly enabled NMI Watchdog Timer interface).exclude_user If this bit is set, the count excludes events that happen in user space.exclude_kernel If this bit is set, the count excludes events that happen in kernel space.exclude_hv If this bit is set, the count excludes events that happen in the hypervisor. This is mainly for PMUs that have built-in support for handling this (such as POWER). Extra support is needed for handling hypervisor measurements on most machines.exclude_idle If set, don't count when the CPU is running the idle task. While you can currently enable this for any event type, it is ignored for all but software events.mmap Themmap bit enables generation ofPERF_RECORD_MMAPsamples for everymmap(2) call that hasPROT_EXECset. This allows tools to notice new executable code being mapped into a program (dynamic shared libraries for example) so that addresses can be mapped back to the original code.comm Thecomm bit enables tracking of process command name as modified by theexecve(2) andprctl(PR_SET_NAME) system calls as well as writing to/proc/self/comm. If thecomm_exec flag is also successfully set (possible since Linux 3.16), then the misc flagPERF_RECORD_MISC_COMM_EXEC can be used to differentiate theexecve(2) case from the others.freq If this bit is set, thensample_frequency notsample_period is used when setting up the sampling interval.inherit_stat This bit enables saving of event counts on context switch for inherited tasks. This is meaningful only if theinherit field is set.enable_on_exec If this bit is set, a counter is automatically enabled after a call toexecve(2).task If this bit is set, then fork/exit notifications are included in the ring buffer.watermark If set, have an overflow notification happen when we cross thewakeup_watermark boundary. Otherwise, overflow notifications happen afterwakeup_events samples.precise_ip (since Linux 2.6.35) This controls the amount of skid. Skid is how many instructions execute between an event of interest happening and the kernel being able to stop and record the event. Smaller skid is better and allows more accurate reporting of which events correspond to which instructions, but hardware is often limited with how small this can be. The possible values of this field are the following:0 SAMPLE_IPcan have arbitrary skid.1 SAMPLE_IPmust have constant skid.2 SAMPLE_IPrequested to have 0 skid.3 SAMPLE_IPmust have 0 skid. See also the description ofPERF_RECORD_MISC_EXACT_IP.mmap_data (since Linux 2.6.36) This is the counterpart of themmap field. This enables generation ofPERF_RECORD_MMAPsamples formmap(2) calls that do not havePROT_EXECset (for example data and SysV shared memory).sample_id_all (since Linux 2.6.38) If set, then TID, TIME, ID, STREAM_ID, and CPU can additionally be included in non-PERF_RECORD_SAMPLEs if the correspondingsample_type is selected. IfPERF_SAMPLE_IDENTIFIERis specified, then an additional ID value is included as the last value to ease parsing the record stream. This may lead to theid value appearing twice. The layout is described by this pseudo-structure: struct sample_id { { u32 pid, tid; } /* if PERF_SAMPLE_TID set */ { u64 time; } /* if PERF_SAMPLE_TIME set */ { u64 id; } /* if PERF_SAMPLE_ID set */ { u64 stream_id;} /* if PERF_SAMPLE_STREAM_ID set */ { u32 cpu, res; } /* if PERF_SAMPLE_CPU set */ { u64 id; } /* if PERF_SAMPLE_IDENTIFIER set */ };exclude_host (since Linux 3.2) When conducting measurements that include processes running VM instances (i.e., have executed aKVM_RUN ioctl(2)), only measure events happening inside a guest instance. This is only meaningful outside the guests; this setting does not change counts gathered inside of a guest. Currently, this functionality is x86 only.exclude_guest (since Linux 3.2) When conducting measurements that include processes running VM instances (i.e., have executed aKVM_RUN ioctl(2)), do not measure events happening inside guest instances. This is only meaningful outside the guests; this setting does not change counts gathered inside of a guest. Currently, this functionality is x86 only.exclude_callchain_kernel (since Linux 3.7) Do not include kernel callchains.exclude_callchain_user (since Linux 3.7) Do not include user callchains.mmap2 (since Linux 3.16) Generate an extended executable mmap record that contains enough additional information to uniquely identify shared mappings. Themmap flag must also be set for this to work.comm_exec (since Linux 3.16) This is purely a feature-detection flag, it does not change kernel behavior. If this flag can successfully be set, then, whencomm is enabled, thePERF_RECORD_MISC_COMM_EXEC flag will be set in themisc field of a comm record header if the rename event being reported was caused by a call toexecve(2). This allows tools to distinguish between the various types of process renaming.use_clockid (since Linux 4.1) This allows selecting which internal Linux clock to use when generating timestamps via theclockid field. This can make it easier to correlate perf sample times with timestamps generated by other tools.context_switch (since Linux 4.3) This enables the generation ofPERF_RECORD_SWITCHrecords when a context switch occurs. It also enables the generation ofPERF_RECORD_SWITCH_CPU_WIDErecords when sampling in CPU-wide mode. This functionality is in addition to existing tracepoint and software events for measuring context switches. The advantage of this method is that it will give full information even with strictperf_event_paranoid settings.write_backward (since Linux 4.6) This causes the ring buffer to be written from the end to the beginning. This is to support reading from overwritable ring buffer.namespaces (since Linux 4.11) This enables the generation ofPERF_RECORD_NAMESPACES records when a task enters a new namespace. Each namespace has a combination of device and inode numbers.ksymbol (since Linux 5.0) This enables the generation ofPERF_RECORD_KSYMBOLrecords when new kernel symbols are registered or unregistered. This is analyzing dynamic kernel functions like eBPF.bpf_event (since Linux 5.0) This enables the generation ofPERF_RECORD_BPF_EVENT records when an eBPF program is loaded or unloaded.aux_output (since Linux 5.4) This allows normal (non-AUX) events to generate data for AUX events if the hardware supports it.cgroup (since Linux 5.7) This enables the generation ofPERF_RECORD_CGROUPrecords when a new cgroup is created (and activated).text_poke (since Linux 5.8) This enables the generation ofPERF_RECORD_TEXT_POKE records when there's a change to the kernel text (i.e., self-modifying code).build_id (since Linux 5.12) This changes the contents in thePERF_RECORD_MMAP2to have a build-id instead of device and inode numbers.inherit_thread (since Linux 5.13) This disables the inheritance of the event to a child process. Only new threads in the same process (which is cloned withCLONE_THREAD) will inherit the event.remove_on_exec (since Linux 5.13) This closes the event when it starts a new process image byexecve(2).sigtrap (since Linux 5.13) This enables synchronous signal delivery ofSIGTRAPon event overflow.wakeup_eventswakeup_watermark This union sets how many samples (wakeup_events) or bytes (wakeup_watermark) happen before an overflow notification happens. Which one is used is selected by thewatermark bit flag.wakeup_events counts onlyPERF_RECORD_SAMPLErecord types. To receive overflow notification for allPERF_RECORDtypes choose watermark and setwakeup_watermark to 1. Prior to Linux 3.0, settingwakeup_events to 0 resulted in no overflow notifications; more recent kernels treat 0 the same as 1.bp_type (since Linux 2.6.33) This chooses the breakpoint type. It is one of:HW_BREAKPOINT_EMPTY No breakpoint.HW_BREAKPOINT_R Count when we read the memory location.HW_BREAKPOINT_W Count when we write the memory location.HW_BREAKPOINT_RW Count when we read or write the memory location.HW_BREAKPOINT_X Count when we execute code at the memory location. The values can be combined via a bitwise or, but the combination ofHW_BREAKPOINT_RorHW_BREAKPOINT_WwithHW_BREAKPOINT_Xis not allowed.bp_addr (since Linux 2.6.33) This is the address of the breakpoint. For execution breakpoints, this is the memory address of the instruction of interest; for read and write breakpoints, it is the memory address of the memory location of interest.config1 (since Linux 2.6.39)config1 is used for setting events that need an extra register or otherwise do not fit in the regular config field. Raw OFFCORE_EVENTS on Nehalem/Westmere/SandyBridge use this field on Linux 3.3 and later kernels.bp_len (since Linux 2.6.33)bp_len is the size of the breakpoint being measured iftype isPERF_TYPE_BREAKPOINT. Options areHW_BREAKPOINT_LEN_1,HW_BREAKPOINT_LEN_2,HW_BREAKPOINT_LEN_4, andHW_BREAKPOINT_LEN_8. For an execution breakpoint, set this tosizeof(long).config2 (since Linux 2.6.39)config2 is a further extension of theconfig1 field.branch_sample_type (since Linux 3.4) IfPERF_SAMPLE_BRANCH_STACKis enabled, then this specifies what branches to include in the branch record. The first part of the value is the privilege level, which is a combination of one of the values listed below. If the user does not set privilege level explicitly, the kernel will use the event's privilege level. Event and branch privilege levels do not have to match.PERF_SAMPLE_BRANCH_USER Branch target is in user space.PERF_SAMPLE_BRANCH_KERNEL Branch target is in kernel space.PERF_SAMPLE_BRANCH_HV Branch target is in hypervisor.PERF_SAMPLE_BRANCH_PLM_ALL A convenience value that is the three preceding values ORed together. In addition to the privilege value, at least one or more of the following bits must be set.PERF_SAMPLE_BRANCH_ANY Any branch type.PERF_SAMPLE_BRANCH_ANY_CALL Any call branch (includes direct calls, indirect calls, and far jumps).PERF_SAMPLE_BRANCH_IND_CALL Indirect calls.PERF_SAMPLE_BRANCH_CALL(since Linux 4.4) Direct calls.PERF_SAMPLE_BRANCH_ANY_RETURN Any return branch.PERF_SAMPLE_BRANCH_IND_JUMP(since Linux 4.2) Indirect jumps.PERF_SAMPLE_BRANCH_COND(since Linux 3.16) Conditional branches.PERF_SAMPLE_BRANCH_ABORT_TX(since Linux 3.11) Transactional memory aborts.PERF_SAMPLE_BRANCH_IN_TX(since Linux 3.11) Branch in transactional memory transaction.PERF_SAMPLE_BRANCH_NO_TX(since Linux 3.11) Branch not in transactional memory transaction.PERF_SAMPLE_BRANCH_CALL_STACK(since Linux 4.1) Branch is part of a hardware-generated call stack. This requires hardware support, currently only found on Intel x86 Haswell or newer.sample_regs_user (since Linux 3.7) This bit mask defines the set of user CPU registers to dump on samples. The layout of the register mask is architecture-specific and is described in the kernel header filearch/ARCH/include/uapi/asm/perf_regs.h.sample_stack_user (since Linux 3.7) This defines the size of the user stack to dump ifPERF_SAMPLE_STACK_USERis specified.clockid (since Linux 4.1) Ifuse_clockid is set, then this field selects which internal Linux timer to use for timestamps. The available timers are defined inlinux/time.h, withCLOCK_MONOTONIC,CLOCK_MONOTONIC_RAW,CLOCK_REALTIME,CLOCK_BOOTTIME, andCLOCK_TAIcurrently supported.aux_watermark (since Linux 4.1) This specifies how much data is required to trigger aPERF_RECORD_AUXsample.sample_max_stack (since Linux 4.8) Whensample_type includesPERF_SAMPLE_CALLCHAIN, this field specifies how many stack frames to report when generating the callchain.aux_sample_size (since Linux 5.5) WhenPERF_SAMPLE_AUXflag is set, specify the desired size of AUX data. Note that it can get smaller data than the specified size.sig_data (since Linux 5.13) This data will be copied to user's signal handler (throughsi_perf in thesiginfo_t) to disambiguate which event triggered the signal.Reading results Once aperf_event_open() file descriptor has been opened, the values of the events can be read from the file descriptor. The values that are there are specified by theread_format field in theattr structure at open time. If you attempt to read into a buffer that is not big enough to hold the data, the errorENOSPCresults. Here is the layout of the data returned by a read: • IfPERF_FORMAT_GROUPwas specified to allow reading all events in a group at once: struct read_format { u64 nr; /* The number of events */ u64 time_enabled; /* if PERF_FORMAT_TOTAL_TIME_ENABLED */ u64 time_running; /* if PERF_FORMAT_TOTAL_TIME_RUNNING */ struct { u64 value; /* The value of the event */ u64 id; /* if PERF_FORMAT_ID */ u64 lost; /* if PERF_FORMAT_LOST */ } values[nr]; }; • IfPERF_FORMAT_GROUPwasnot specified: struct read_format { u64 value; /* The value of the event */ u64 time_enabled; /* if PERF_FORMAT_TOTAL_TIME_ENABLED */ u64 time_running; /* if PERF_FORMAT_TOTAL_TIME_RUNNING */ u64 id; /* if PERF_FORMAT_ID */ u64 lost; /* if PERF_FORMAT_LOST */ }; The values read are as follows:nr The number of events in this file descriptor. Available only ifPERF_FORMAT_GROUPwas specified.time_enabledtime_running Total time the event was enabled and running. Normally these values are the same. Multiplexing happens if the number of events is more than the number of available PMU counter slots. In that case the events run only part of the time and thetime_enabled andtime running values can be used to scale an estimated value for the count.value An unsigned 64-bit value containing the counter result.id A globally unique value for this particular event; only present ifPERF_FORMAT_IDwas specified inread_format.lost The number of lost samples of this event; only present ifPERF_FORMAT_LOSTwas specified inread_format.MMAP layout When usingperf_event_open() in sampled mode, asynchronous events (like counter overflow orPROT_EXECmmap tracking) are logged into a ring-buffer. This ring-buffer is created and accessed throughmmap(2). The mmap size should be 1+2^n pages, where the first page is a metadata page (struct perf_event_mmap_page) that contains various bits of information such as where the ring-buffer head is. Before Linux 2.6.39, there is a bug that means you must allocate an mmap ring buffer when sampling even if you do not plan to access it. The structure of the first metadata mmap page is as follows: struct perf_event_mmap_page { __u32 version; /* version number of this structure */ __u32 compat_version; /* lowest version this is compat with */ __u32 lock; /* seqlock for synchronization */ __u32 index; /* hardware counter identifier */ __s64 offset; /* add to hardware counter value */ __u64 time_enabled; /* time event active */ __u64 time_running; /* time event on CPU */ union { __u64 capabilities; struct { __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1, cap_bit0_is_deprecated : 1, cap_user_rdpmc : 1, cap_user_time : 1, cap_user_time_zero : 1, }; }; __u16 pmc_width; __u16 time_shift; __u32 time_mult; __u64 time_offset; __u64 __reserved[120]; /* Pad to 1 k */ __u64 data_head; /* head in the data section */ __u64 data_tail; /* user-space written tail */ __u64 data_offset; /* where the buffer starts */ __u64 data_size; /* data buffer size */ __u64 aux_head; __u64 aux_tail; __u64 aux_offset; __u64 aux_size; } The following list describes the fields in theperf_event_mmap_page structure in more detail:version Version number of this structure.compat_version The lowest version this is compatible with.lock A seqlock for synchronization.index A unique hardware counter identifier.offset When using rdpmc for reads this offset value must be added to the one returned by rdpmc to get the current total event count.time_enabled Time the event was active.time_running Time the event was running.cap_usr_time /cap_usr_rdpmc /cap_bit0 (since Linux 3.4) There was a bug in the definition ofcap_usr_time andcap_usr_rdpmc from Linux 3.4 until Linux 3.11. Both bits were defined to point to the same location, so it was impossible to know ifcap_usr_time orcap_usr_rdpmc were actually set. Starting with Linux 3.12, these are renamed tocap_bit0 and you should use thecap_user_time andcap_user_rdpmc fields instead.cap_bit0_is_deprecated (since Linux 3.12) If set, this bit indicates that the kernel supports the properly separatedcap_user_time andcap_user_rdpmc bits. If not-set, it indicates an older kernel wherecap_usr_time andcap_usr_rdpmc map to the same bit and thus both features should be used with caution.cap_user_rdpmc (since Linux 3.12) If the hardware supports user-space read of performance counters without syscall (this is the "rdpmc" instruction on x86), then the following code can be used to do a read: u32 seq, time_mult, time_shift, idx, width; u64 count, enabled, running; u64 cyc, time_offset; do { seq = pc->lock; barrier(); enabled = pc->time_enabled; running = pc->time_running; if (pc->cap_usr_time && enabled != running) { cyc = rdtsc(); time_offset = pc->time_offset; time_mult = pc->time_mult; time_shift = pc->time_shift; } idx = pc->index; count = pc->offset; if (pc->cap_usr_rdpmc && idx) { width = pc->pmc_width; count += rdpmc(idx - 1); } barrier(); } while (pc->lock != seq);cap_user_time (since Linux 3.12) This bit indicates the hardware has a constant, nonstop timestamp counter (TSC on x86).cap_user_time_zero (since Linux 3.12) Indicates the presence oftime_zero which allows mapping timestamp values to the hardware clock.pmc_width Ifcap_usr_rdpmc, this field provides the bit-width of the value read using the rdpmc or equivalent instruction. This can be used to sign extend the result like: pmc <<= 64 - pmc_width; pmc >>= 64 - pmc_width; // signed shift right count += pmc;time_shifttime_multtime_offset Ifcap_usr_time, these fields can be used to compute the time delta sincetime_enabled (in nanoseconds) using rdtsc or similar. u64 quot, rem; u64 delta; quot = cyc >> time_shift; rem = cyc & (((u64)1 << time_shift) - 1); delta = time_offset + quot * time_mult + ((rem * time_mult) >> time_shift); Wheretime_offset,time_mult,time_shift, andcyc are read in the seqcount loop described above. This delta can then be added to enabled and possible running (if idx), improving the scaling: enabled += delta; if (idx) running += delta; quot = count / running; rem = count % running; count = quot * enabled + (rem * enabled) / running;time_zero (since Linux 3.12) Ifcap_usr_time_zero is set, then the hardware clock (the TSC timestamp counter on x86) can be calculated from thetime_zero,time_mult, andtime_shift values: time = timestamp - time_zero; quot = time / time_mult; rem = time % time_mult; cyc = (quot << time_shift) + (rem << time_shift) / time_mult; And vice versa: quot = cyc >> time_shift; rem = cyc & (((u64)1 << time_shift) - 1); timestamp = time_zero + quot * time_mult + ((rem * time_mult) >> time_shift);data_head This points to the head of the data section. The value continuously increases, it does not wrap. The value needs to be manually wrapped by the size of the mmap buffer before accessing the samples. On SMP-capable platforms, after reading thedata_head value, user space should issue an rmb().data_tail When the mapping isPROT_WRITE, thedata_tail value should be written by user space to reflect the last read data. In this case, the kernel will not overwrite unread data.data_offset (since Linux 4.1) Contains the offset of the location in the mmap buffer where perf sample data begins.data_size (since Linux 4.1) Contains the size of the perf sample region within the mmap buffer.aux_headaux_tailaux_offsetaux_size(since Linux 4.1) The AUX region allowsmmap(2)-ing a separate sample buffer for high-bandwidth data streams (separate from the main perf sample buffer). An example of a high-bandwidth stream is instruction tracing support, as is found in newer Intel processors. To set up an AUX area, firstaux_offset needs to be set with an offset greater thandata_offset+data_size andaux_size needs to be set to the desired buffer size. The desired offset and size must be page aligned, and the size must be a power of two. These values are then passed to mmap in order to map the AUX buffer. Pages in the AUX buffer are included as part of theRLIMIT_MEMLOCKresource limit (seesetrlimit(2)), and also as part of theperf_event_mlock_kb allowance. By default, the AUX buffer will be truncated if it will not fit in the available space in the ring buffer. If the AUX buffer is mapped as a read only buffer, then it will operate in ring buffer mode where old data will be overwritten by new. In overwrite mode, it might not be possible to infer where the new data began, and it is the consumer's job to disable measurement while reading to avoid possible data races. Theaux_head andaux_tail ring buffer pointers have the same behavior and ordering rules as the previous describeddata_head anddata_tail. The following 2^n ring-buffer pages have the layout described below. Ifperf_event_attr.sample_id_all is set, then all event types will have the sample_type selected fields related to where/when (identity) an event took place (TID, TIME, ID, CPU, STREAM_ID) described inPERF_RECORD_SAMPLEbelow, it will be stashed just after theperf_event_header and the fields already present for the existing fields, that is, at the end of the payload. This allows a newer perf.data file to be supported by older perf tools, with the new optional fields being ignored. The mmap values start with a header: struct perf_event_header { __u32 type; __u16 misc; __u16 size; }; Below, we describe theperf_event_header fields in more detail. For ease of reading, the fields with shorter descriptions are presented first.size This indicates the size of the record.misc Themisc field contains additional information about the sample. The CPU mode can be determined from this value by masking withPERF_RECORD_MISC_CPUMODE_MASKand looking for one of the following (note these are not bit masks, only one can be set at a time):PERF_RECORD_MISC_CPUMODE_UNKNOWN Unknown CPU mode.PERF_RECORD_MISC_KERNEL Sample happened in the kernel.PERF_RECORD_MISC_USER Sample happened in user code.PERF_RECORD_MISC_HYPERVISOR Sample happened in the hypervisor.PERF_RECORD_MISC_GUEST_KERNEL(since Linux 2.6.35) Sample happened in the guest kernel.PERF_RECORD_MISC_GUEST_USER (since Linux 2.6.35) Sample happened in guest user code. Since the following three statuses are generated by different record types, they alias to the same bit:PERF_RECORD_MISC_MMAP_DATA(since Linux 3.10) This is set when the mapping is not executable; otherwise the mapping is executable.PERF_RECORD_MISC_COMM_EXEC(since Linux 3.16) This is set for aPERF_RECORD_COMMrecord on kernels more recent than Linux 3.16 if a process name change was caused by anexecve(2) system call.PERF_RECORD_MISC_SWITCH_OUT(since Linux 4.3) When aPERF_RECORD_SWITCHorPERF_RECORD_SWITCH_CPU_WIDErecord is generated, this bit indicates that the context switch is away from the current process (instead of into the current process). In addition, the following bits can be set:PERF_RECORD_MISC_EXACT_IP This indicates that the content ofPERF_SAMPLE_IP points to the actual instruction that triggered the event. See alsoperf_event_attr.precise_ip.PERF_RECORD_MISC_SWITCH_OUT_PREEMPT(since Linux 4.17) When aPERF_RECORD_SWITCHorPERF_RECORD_SWITCH_CPU_WIDErecord is generated, this indicates the context switch was a preemption.PERF_RECORD_MISC_MMAP_BUILD_ID(since Linux 5.12) This indicates that the content ofPERF_SAMPLE_MMAP2 contains build-ID data instead of device major and minor numbers as well as the inode number.PERF_RECORD_MISC_EXT_RESERVED(since Linux 2.6.35) This indicates there is extended data available (currently not used).PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT This bit is not set by the kernel. It is reserved for the user-space perf utility to indicate that/proc/pid/maps parsing was taking too long and was stopped, and thus the mmap records may be truncated.type Thetype value is one of the below. The values in the corresponding record (that follows the header) depend on thetype selected as shown.PERF_RECORD_MMAP The MMAP events record thePROT_EXECmappings so that we can correlate user-space IPs to code. They have the following structure: struct { struct perf_event_header header; u32 pid, tid; u64 addr; u64 len; u64 pgoff; char filename[]; };pid is the process ID.tid is the thread ID.addr is the address of the allocated memory.len is the size of the allocated memory.pgoff is the page offset of the allocated memory.filename is a string describing the backing of the allocated memory.PERF_RECORD_LOST This record indicates when events are lost. struct { struct perf_event_header header; u64 id; u64 lost; struct sample_id sample_id; };id is the unique event ID for the samples that were lost.lost is the number of events that were lost.PERF_RECORD_COMM This record indicates a change in the process name. struct { struct perf_event_header header; u32 pid; u32 tid; char comm[]; struct sample_id sample_id; };pid is the process ID.tid is the thread ID.comm is a string containing the new name of the process.PERF_RECORD_EXIT This record indicates a process exit event. struct { struct perf_event_header header; u32 pid, ppid; u32 tid, ptid; u64 time; struct sample_id sample_id; };PERF_RECORD_THROTTLEPERF_RECORD_UNTHROTTLE This record indicates a throttle/unthrottle event. struct { struct perf_event_header header; u64 time; u64 id; u64 stream_id; struct sample_id sample_id; };PERF_RECORD_FORK This record indicates a fork event. struct { struct perf_event_header header; u32 pid, ppid; u32 tid, ptid; u64 time; struct sample_id sample_id; };PERF_RECORD_READ This record indicates a read event. struct { struct perf_event_header header; u32 pid, tid; struct read_format values; struct sample_id sample_id; };PERF_RECORD_SAMPLE This record indicates a sample. struct { struct perf_event_header header; u64 sample_id; /* if PERF_SAMPLE_IDENTIFIER */ u64 ip; /* if PERF_SAMPLE_IP */ u32 pid, tid; /* if PERF_SAMPLE_TID */ u64 time; /* if PERF_SAMPLE_TIME */ u64 addr; /* if PERF_SAMPLE_ADDR */ u64 id; /* if PERF_SAMPLE_ID */ u64 stream_id; /* if PERF_SAMPLE_STREAM_ID */ u32 cpu, res; /* if PERF_SAMPLE_CPU */ u64 period; /* if PERF_SAMPLE_PERIOD */ struct read_format v; /* if PERF_SAMPLE_READ */ u64 nr; /* if PERF_SAMPLE_CALLCHAIN */ u64 ips[nr]; /* if PERF_SAMPLE_CALLCHAIN */ u32 size; /* if PERF_SAMPLE_RAW */ char data[size]; /* if PERF_SAMPLE_RAW */ u64 bnr; /* if PERF_SAMPLE_BRANCH_STACK */ struct perf_branch_entry lbr[bnr]; /* if PERF_SAMPLE_BRANCH_STACK */ u64 abi; /* if PERF_SAMPLE_REGS_USER */ u64 regs[weight(mask)]; /* if PERF_SAMPLE_REGS_USER */ u64 size; /* if PERF_SAMPLE_STACK_USER */ char data[size]; /* if PERF_SAMPLE_STACK_USER */ u64 dyn_size; /* if PERF_SAMPLE_STACK_USER && size != 0 */ union perf_sample_weight weight; /* if PERF_SAMPLE_WEIGHT */ /* || PERF_SAMPLE_WEIGHT_STRUCT */ u64 data_src; /* if PERF_SAMPLE_DATA_SRC */ u64 transaction; /* if PERF_SAMPLE_TRANSACTION */ u64 abi; /* if PERF_SAMPLE_REGS_INTR */ u64 regs[weight(mask)]; /* if PERF_SAMPLE_REGS_INTR */ u64 phys_addr; /* if PERF_SAMPLE_PHYS_ADDR */ u64 cgroup; /* if PERF_SAMPLE_CGROUP */ u64 data_page_size; /* if PERF_SAMPLE_DATA_PAGE_SIZE */ u64 code_page_size; /* if PERF_SAMPLE_CODE_PAGE_SIZE */ u64 size; /* if PERF_SAMPLE_AUX */ char data[size]; /* if PERF_SAMPLE_AUX */ };sample_id IfPERF_SAMPLE_IDENTIFIERis enabled, a 64-bit unique ID is included. This is a duplication of thePERF_SAMPLE_IDid value, but included at the beginning of the sample so parsers can easily obtain the value.ip IfPERF_SAMPLE_IPis enabled, then a 64-bit instruction pointer value is included.pidtid IfPERF_SAMPLE_TIDis enabled, then a 32-bit process ID and 32-bit thread ID are included.time IfPERF_SAMPLE_TIMEis enabled, then a 64-bit timestamp is included. This is obtained via local_clock() which is a hardware timestamp if available and the jiffies value if not.addr IfPERF_SAMPLE_ADDRis enabled, then a 64-bit address is included. This is usually the address of a tracepoint, breakpoint, or software event; otherwise the value is 0.id IfPERF_SAMPLE_IDis enabled, a 64-bit unique ID is included. If the event is a member of an event group, the group leader ID is returned. This ID is the same as the one returned byPERF_FORMAT_ID.stream_id IfPERF_SAMPLE_STREAM_IDis enabled, a 64-bit unique ID is included. UnlikePERF_SAMPLE_IDthe actual ID is returned, not the group leader. This ID is the same as the one returned byPERF_FORMAT_ID.cpures IfPERF_SAMPLE_CPUis enabled, this is a 32-bit value indicating which CPU was being used, in addition to a reserved (unused) 32-bit value.period IfPERF_SAMPLE_PERIODis enabled, a 64-bit value indicating the current sampling period is written.v IfPERF_SAMPLE_READis enabled, a structure of type read_format is included which has values for all events in the event group. The values included depend on theread_format value used atperf_event_open() time.nrips[nr] IfPERF_SAMPLE_CALLCHAINis enabled, then a 64-bit number is included which indicates how many following 64-bit instruction pointers will follow. This is the current callchain.sizedata[size] IfPERF_SAMPLE_RAWis enabled, then a 32-bit value indicating size is included followed by an array of 8-bit values of sizesize. The values are padded with 0 to have 64-bit alignment. This RAW record data is opaque with respect to the ABI. The ABI doesn't make any promises with respect to the stability of its content, it may vary depending on event, hardware, and kernel version.bnrlbr[bnr] IfPERF_SAMPLE_BRANCH_STACKis enabled, then a 64-bit value indicating the number of records is included, followed bybnr perf_branch_entry structures which each include the fields:from This indicates the source instruction (may not be a branch).to The branch target.mispred The branch target was mispredicted.predicted The branch target was predicted.in_tx (since Linux 3.11) The branch was in a transactional memory transaction.abort (since Linux 3.11) The branch was in an aborted transactional memory transaction.cycles (since Linux 4.3) This reports the number of cycles elapsed since the previous branch stack update. The entries are from most to least recent, so the first entry has the most recent branch. Support formispred,predicted, andcycles is optional; if not supported, those values will be 0. The type of branches recorded is specified by thebranch_sample_type field.abiregs[weight(mask)] IfPERF_SAMPLE_REGS_USERis enabled, then the user CPU registers are recorded. Theabi field is one ofPERF_SAMPLE_REGS_ABI_NONE,PERF_SAMPLE_REGS_ABI_32, orPERF_SAMPLE_REGS_ABI_64. Theregs field is an array of the CPU registers that were specified by thesample_regs_user attr field. The number of values is the number of bits set in thesample_regs_user bit mask.sizedata[size]dyn_size IfPERF_SAMPLE_STACK_USERis enabled, then the user stack is recorded. This can be used to generate stack backtraces.size is the size requested by the user insample_stack_user or else the maximum record size.data is the stack data (a raw dump of the memory pointed to by the stack pointer at the time of sampling).dyn_size is the amount of data actually dumped (can be less thansize). Note thatdyn_size is omitted ifsize is 0.weight IfPERF_SAMPLE_WEIGHTorPERF_SAMPLE_WEIGHT_STRUCT is enabled, then a 64-bit value provided by the hardware is recorded that indicates how costly the event was. This allows expensive events to stand out more clearly in profiles.data_src IfPERF_SAMPLE_DATA_SRCis enabled, then a 64-bit value is recorded that is made up of the following fields:mem_op Type of opcode, a bitwise combination of:PERF_MEM_OP_NA Not availablePERF_MEM_OP_LOAD Load instructionPERF_MEM_OP_STORE Store instructionPERF_MEM_OP_PFETCH PrefetchPERF_MEM_OP_EXEC Executable codemem_lvl Memory hierarchy level hit or miss, a bitwise combination of the following, shifted left byPERF_MEM_LVL_SHIFT:PERF_MEM_LVL_NA Not availablePERF_MEM_LVL_HIT HitPERF_MEM_LVL_MISS MissPERF_MEM_LVL_L1 Level 1 cachePERF_MEM_LVL_LFB Line fill bufferPERF_MEM_LVL_L2 Level 2 cachePERF_MEM_LVL_L3 Level 3 cachePERF_MEM_LVL_LOC_RAM Local DRAMPERF_MEM_LVL_REM_RAM1 Remote DRAM 1 hopPERF_MEM_LVL_REM_RAM2 Remote DRAM 2 hopsPERF_MEM_LVL_REM_CCE1 Remote cache 1 hopPERF_MEM_LVL_REM_CCE2 Remote cache 2 hopsPERF_MEM_LVL_IO I/O memoryPERF_MEM_LVL_UNC Uncached memorymem_snoop Snoop mode, a bitwise combination of the following, shifted left byPERF_MEM_SNOOP_SHIFT:PERF_MEM_SNOOP_NA Not availablePERF_MEM_SNOOP_NONE No snoopPERF_MEM_SNOOP_HIT Snoop hitPERF_MEM_SNOOP_MISS Snoop missPERF_MEM_SNOOP_HITM Snoop hit modifiedmem_lock Lock instruction, a bitwise combination of the following, shifted left byPERF_MEM_LOCK_SHIFT:PERF_MEM_LOCK_NA Not availablePERF_MEM_LOCK_LOCKED Locked transactionmem_dtlb TLB access hit or miss, a bitwise combination of the following, shifted left byPERF_MEM_TLB_SHIFT:PERF_MEM_TLB_NA Not availablePERF_MEM_TLB_HIT HitPERF_MEM_TLB_MISS MissPERF_MEM_TLB_L1 Level 1 TLBPERF_MEM_TLB_L2 Level 2 TLBPERF_MEM_TLB_WK Hardware walkerPERF_MEM_TLB_OS OS fault handlertransaction If thePERF_SAMPLE_TRANSACTIONflag is set, then a 64-bit field is recorded describing the sources of any transactional memory aborts. The field is a bitwise combination of the following values:PERF_TXN_ELISION Abort from an elision type transaction (Intel-CPU-specific).PERF_TXN_TRANSACTION Abort from a generic transaction.PERF_TXN_SYNC Synchronous abort (related to the reported instruction).PERF_TXN_ASYNC Asynchronous abort (not related to the reported instruction).PERF_TXN_RETRY Retryable abort (retrying the transaction may have succeeded).PERF_TXN_CONFLICT Abort due to memory conflicts with other threads.PERF_TXN_CAPACITY_WRITE Abort due to write capacity overflow.PERF_TXN_CAPACITY_READ Abort due to read capacity overflow. In addition, a user-specified abort code can be obtained from the high 32 bits of the field by shifting right byPERF_TXN_ABORT_SHIFTand masking with the valuePERF_TXN_ABORT_MASK.abiregs[weight(mask)] IfPERF_SAMPLE_REGS_INTRis enabled, then the user CPU registers are recorded. Theabi field is one ofPERF_SAMPLE_REGS_ABI_NONE,PERF_SAMPLE_REGS_ABI_32, orPERF_SAMPLE_REGS_ABI_64. Theregs field is an array of the CPU registers that were specified by thesample_regs_intr attr field. The number of values is the number of bits set in thesample_regs_intr bit mask.phys_addr If thePERF_SAMPLE_PHYS_ADDRflag is set, then the 64-bit physical address is recorded.cgroup If thePERF_SAMPLE_CGROUPflag is set, then the 64-bit cgroup ID (for the perf_event subsystem) is recorded. To get the pathname of the cgroup, the ID should match to one in aPERF_RECORD_CGROUP.data_page_size If thePERF_SAMPLE_DATA_PAGE_SIZEflag is set, then the 64-bit page size value of thedataaddress is recorded.code_page_size If thePERF_SAMPLE_CODE_PAGE_SIZEflag is set, then the 64-bit page size value of theipaddress is recorded.sizedata[size] IfPERF_SAMPLE_AUXis enabled, a snapshot of the aux buffer is recorded.PERF_RECORD_MMAP2 This record includes extended information onmmap(2) calls returning executable mappings. The format is similar to that of thePERF_RECORD_MMAPrecord, but includes extra values that allow uniquely identifying shared mappings. Depending on thePERF_RECORD_MISC_MMAP_BUILD_IDbit in the header, the extra values have different layout and meanings. struct { struct perf_event_header header; u32 pid; u32 tid; u64 addr; u64 len; u64 pgoff; union { struct { u32 maj; u32 min; u64 ino; u64 ino_generation; }; struct { /* if PERF_RECORD_MISC_MMAP_BUILD_ID */ u8 build_id_size; u8 __reserved_1; u16 __reserved_2; u8 build_id[20]; }; }; u32 prot; u32 flags; char filename[]; struct sample_id sample_id; };pid is the process ID.tid is the thread ID.addr is the address of the allocated memory.len is the size of the allocated memory.pgoff is the page offset of the allocated memory.maj is the major ID of the underlying device.min is the minor ID of the underlying device.ino is the inode number.ino_generation is the inode generation.build_id_size is the actual size ofbuild_id field (up to 20).build_id is a raw data to identify a binary.prot is the protection information.flags is the flags information.filename is a string describing the backing of the allocated memory.PERF_RECORD_AUX(since Linux 4.1) This record reports that new data is available in the separate AUX buffer region. struct { struct perf_event_header header; u64 aux_offset; u64 aux_size; u64 flags; struct sample_id sample_id; };aux_offset offset in the AUX mmap region where the new data begins.aux_size size of the data made available.flags describes the AUX update.PERF_AUX_FLAG_TRUNCATED if set, then the data returned was truncated to fit the available buffer size.PERF_AUX_FLAG_OVERWRITE if set, then the data returned has overwritten previous data.PERF_RECORD_ITRACE_START(since Linux 4.1) This record indicates which process has initiated an instruction trace event, allowing tools to properly correlate the instruction addresses in the AUX buffer with the proper executable. struct { struct perf_event_header header; u32 pid; u32 tid; };pid process ID of the thread starting an instruction trace.tid thread ID of the thread starting an instruction trace.PERF_RECORD_LOST_SAMPLES(since Linux 4.2) When using hardware sampling (such as Intel PEBS) this record indicates some number of samples that may have been lost. struct { struct perf_event_header header; u64 lost; struct sample_id sample_id; };lost the number of potentially lost samples.PERF_RECORD_SWITCH(since Linux 4.3) This record indicates a context switch has happened. ThePERF_RECORD_MISC_SWITCH_OUTbit in themisc field indicates whether it was a context switch into or away from the current process. struct { struct perf_event_header header; struct sample_id sample_id; };PERF_RECORD_SWITCH_CPU_WIDE(since Linux 4.3) As withPERF_RECORD_SWITCHthis record indicates a context switch has happened, but it only occurs when sampling in CPU-wide mode and provides additional information on the process being switched to/from. ThePERF_RECORD_MISC_SWITCH_OUTbit in themisc field indicates whether it was a context switch into or away from the current process. struct { struct perf_event_header header; u32 next_prev_pid; u32 next_prev_tid; struct sample_id sample_id; };next_prev_pid The process ID of the previous (if switching in) or next (if switching out) process on the CPU.next_prev_tid The thread ID of the previous (if switching in) or next (if switching out) thread on the CPU.PERF_RECORD_NAMESPACES(since Linux 4.11) This record includes various namespace information of a process. struct { struct perf_event_header header; u32 pid; u32 tid; u64 nr_namespaces; struct { u64 dev, inode } [nr_namespaces]; struct sample_id sample_id; };pid is the process IDtid is the thread IDnr_namespace is the number of namespaces in this record Each namespace hasdev andinode fields and is recorded in the fixed position like below:NET_NS_INDEX=0 Network namespaceUTS_NS_INDEX=1 UTS namespaceIPC_NS_INDEX=2 IPC namespacePID_NS_INDEX=3 PID namespaceUSER_NS_INDEX=4 User namespaceMNT_NS_INDEX=5 Mount namespaceCGROUP_NS_INDEX=6 Cgroup namespacePERF_RECORD_KSYMBOL(since Linux 5.0) This record indicates kernel symbol register/unregister events. struct { struct perf_event_header header; u64 addr; u32 len; u16 ksym_type; u16 flags; char name[]; struct sample_id sample_id; };addr is the address of the kernel symbol.len is the size of the kernel symbol.ksym_type is the type of the kernel symbol. Currently the following types are available:PERF_RECORD_KSYMBOL_TYPE_BPF The kernel symbol is a BPF function.flags If thePERF_RECORD_KSYMBOL_FLAGS_UNREGISTERis set, then this event is for unregistering the kernel symbol.PERF_RECORD_BPF_EVENT(since Linux 5.0) This record indicates BPF program is loaded or unloaded. struct { struct perf_event_header header; u16 type; u16 flags; u32 id; u8 tag[BPF_TAG_SIZE]; struct sample_id sample_id; };type is one of the following values:PERF_BPF_EVENT_PROG_LOAD A BPF program is loadedPERF_BPF_EVENT_PROG_UNLOAD A BPF program is unloadedid is the ID of the BPF program.tag is the tag of the BPF program. Currently,BPF_TAG_SIZEis defined as 8.PERF_RECORD_CGROUP(since Linux 5.7) This record indicates a new cgroup is created and activated. struct { struct perf_event_header header; u64 id; char path[]; struct sample_id sample_id; };id is the cgroup identifier. This can be also retrieved byname_to_handle_at(2) on the cgroup path (as a file handle).path is the path of the cgroup from the root.PERF_RECORD_TEXT_POKE(since Linux 5.8) This record indicates a change in the kernel text. This includes addition and removal of the text and the corresponding size is zero in this case. struct { struct perf_event_header header; u64 addr; u16 old_len; u16 new_len; u8 bytes[]; struct sample_id sample_id; };addr is the address of the changeold_len is the old sizenew_len is the new sizebytes contains old bytes immediately followed by new bytes.Overflow handling Events can be set to notify when a threshold is crossed, indicating an overflow. Overflow conditions can be captured by monitoring the event file descriptor withpoll(2),select(2), orepoll(7). Alternatively, the overflow events can be captured via sa signal handler, by enabling I/O signaling on the file descriptor; see the discussion of theF_SETOWNandF_SETSIG operations infcntl(2). Overflows are generated only by sampling events (sample_period must have a nonzero value). There are two ways to generate overflow notifications. The first is to set awakeup_events orwakeup_watermark value that will trigger if a certain number of samples or bytes have been written to the mmap ring buffer. In this case,POLL_INis indicated. The other way is by use of thePERF_EVENT_IOC_REFRESHioctl. This ioctl adds to a counter that decrements each time the event overflows. When nonzero,POLL_INis indicated, but once the counter reaches 0POLL_HUPis indicated and the underlying event is disabled. Refreshing an event group leader refreshes all siblings and refreshing with a parameter of 0 currently enables infinite refreshes; these behaviors are unsupported and should not be relied on. Starting with Linux 3.18,POLL_HUPis indicated if the event being monitored is attached to a different process and that process exits.rdpmc instruction Starting with Linux 3.4 on x86, you can use therdpmc instruction to get low-latency reads without having to enter the kernel. Note that usingrdpmc is not necessarily faster than other methods for reading event values. Support for this can be detected with thecap_usr_rdpmc field in the mmap page; documentation on how to calculate event values can be found in that section. Originally, when rdpmc support was enabled, any process (not just ones with an active perf event) could use the rdpmc instruction to access the counters. Starting with Linux 4.0, rdpmc support is only allowed if an event is currently enabled in a process's context. To restore the old behavior, write the value 2 to/sys/devices/cpu/rdpmc.perf_event ioctl calls Various ioctls act onperf_event_open() file descriptors:PERF_EVENT_IOC_ENABLE This enables the individual event or event group specified by the file descriptor argument. If thePERF_IOC_FLAG_GROUPbit is set in the ioctl argument, then all events in a group are enabled, even if the event specified is not the group leader (but see BUGS).PERF_EVENT_IOC_DISABLE This disables the individual counter or event group specified by the file descriptor argument. Enabling or disabling the leader of a group enables or disables the entire group; that is, while the group leader is disabled, none of the counters in the group will count. Enabling or disabling a member of a group other than the leader affects only that counter; disabling a non-leader stops that counter from counting but doesn't affect any other counter. If thePERF_IOC_FLAG_GROUPbit is set in the ioctl argument, then all events in a group are disabled, even if the event specified is not the group leader (but see BUGS).PERF_EVENT_IOC_REFRESH Non-inherited overflow counters can use this to enable a counter for a number of overflows specified by the argument, after which it is disabled. Subsequent calls of this ioctl add the argument value to the current count. An overflow notification withPOLL_INset will happen on each overflow until the count reaches 0; when that happens a notification withPOLL_HUPset is sent and the event is disabled. Using an argument of 0 is considered undefined behavior.PERF_EVENT_IOC_RESET Reset the event count specified by the file descriptor argument to zero. This resets only the counts; there is no way to reset the multiplexingtime_enabled ortime_running values. If thePERF_IOC_FLAG_GROUPbit is set in the ioctl argument, then all events in a group are reset, even if the event specified is not the group leader (but see BUGS).PERF_EVENT_IOC_PERIOD This updates the overflow period for the event. Since Linux 3.7 (on ARM) and Linux 3.14 (all other architectures), the new period takes effect immediately. On older kernels, the new period did not take effect until after the next overflow. The argument is a pointer to a 64-bit value containing the desired new period. Prior to Linux 2.6.36, this ioctl always failed due to a bug in the kernel.PERF_EVENT_IOC_SET_OUTPUT This tells the kernel to report event notifications to the specified file descriptor rather than the default one. The file descriptors must all be on the same CPU. The argument specifies the desired file descriptor, or -1 if output should be ignored.PERF_EVENT_IOC_SET_FILTER(since Linux 2.6.33) This adds an ftrace filter to this event. The argument is a pointer to the desired ftrace filter.PERF_EVENT_IOC_ID(since Linux 3.12) This returns the event ID value for the given event file descriptor. The argument is a pointer to a 64-bit unsigned integer to hold the result.PERF_EVENT_IOC_SET_BPF(since Linux 4.1) This allows attaching a Berkeley Packet Filter (BPF) program to an existing kprobe tracepoint event. You needCAP_PERFMON(since Linux 5.8) orCAP_SYS_ADMINprivileges to use this ioctl. The argument is a BPF program file descriptor that was created by a previousbpf(2) system call.PERF_EVENT_IOC_PAUSE_OUTPUT(since Linux 4.7) This allows pausing and resuming the event's ring-buffer. A paused ring-buffer does not prevent generation of samples, but simply discards them. The discarded samples are considered lost, and cause aPERF_RECORD_LOSTsample to be generated when possible. An overflow signal may still be triggered by the discarded sample even though the ring- buffer remains empty. The argument is an unsigned 32-bit integer. A nonzero value pauses the ring-buffer, while a zero value resumes the ring-buffer.PERF_EVENT_MODIFY_ATTRIBUTES(since Linux 4.17) This allows modifying an existing event without the overhead of closing and reopening a new event. Currently this is supported only for breakpoint events. The argument is a pointer to aperf_event_attr structure containing the updated event settings.PERF_EVENT_IOC_QUERY_BPF(since Linux 4.16) This allows querying which Berkeley Packet Filter (BPF) programs are attached to an existing kprobe tracepoint. You can only attach one BPF program per event, but you can have multiple events attached to a tracepoint. Querying this value on one tracepoint event returns the ID of all BPF programs in all events attached to the tracepoint. You needCAP_PERFMON(since Linux 5.8) orCAP_SYS_ADMIN privileges to use this ioctl. The argument is a pointer to a structure struct perf_event_query_bpf { __u32 ids_len; __u32 prog_cnt; __u32 ids[0]; }; Theids_len field indicates the number of ids that can fit in the providedids array. Theprog_cnt value is filled in by the kernel with the number of attached BPF programs. Theids array is filled with the ID of each attached BPF program. If there are more programs than will fit in the array, then the kernel will returnENOSPCandids_len will indicate the number of program IDs that were successfully copied.Using prctl(2) A process can enable or disable all currently open event groups using theprctl(2)PR_TASK_PERF_EVENTS_ENABLEandPR_TASK_PERF_EVENTS_DISABLEoperations. This applies only to events created locally by the calling process. This does not apply to events created by other processes attached to the calling process or inherited events from a parent process. Only group leaders are enabled and disabled, not any other members of the groups.perf_event related configuration files Files in/proc/sys/kernel//proc/sys/kernel/perf_event_paranoid Theperf_event_paranoid file can be set to restrict access to the performance counters.2allow only user-space measurements (default since Linux 4.6).1allow both kernel and user measurements (default before Linux 4.6).0allow access to CPU-specific data but not raw tracepoint samples.-1no restrictions. The existence of theperf_event_paranoid file is the official method for determining if a kernel supportsperf_event_open()./proc/sys/kernel/perf_event_max_sample_rate This sets the maximum sample rate. Setting this too high can allow users to sample at a rate that impacts overall machine performance and potentially lock up the machine. The default value is 100000 (samples per second)./proc/sys/kernel/perf_event_max_stack This file sets the maximum depth of stack frame entries reported when generating a call trace./proc/sys/kernel/perf_event_mlock_kb Maximum number of pages an unprivileged user canmlock(2). The default is 516 (kB). Files in/sys/bus/event_source/devices/ Since Linux 2.6.34, the kernel supports having multiple PMUs available for monitoring. Information on how to program these PMUs can be found under/sys/bus/event_source/devices/. Each subdirectory corresponds to a different PMU./sys/bus/event_source/devices/*/type (since Linux 2.6.38) This contains an integer that can be used in thetype field ofperf_event_attr to indicate that you wish to use this PMU./sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4) If this file is 1, then direct user-space access to the performance counter registers is allowed via the rdpmc instruction. This can be disabled by echoing 0 to the file. As of Linux 4.0 the behavior has changed, so that 1 now means only allow access to processes with active perf events, with 2 indicating the old allow-anyone-access behavior./sys/bus/event_source/devices/*/format/ (since Linux 3.4) This subdirectory contains information on the architecture-specific subfields available for programming the variousconfig fields in theperf_event_attr struct. The content of each file is the name of the config field, followed by a colon, followed by a series of integer bit ranges separated by commas. For example, the fileevent may contain the valueconfig1:1,6-10,44 which indicates that event is an attribute that occupies bits 1,6–10, and 44 ofperf_event_attr::config1./sys/bus/event_source/devices/*/events/ (since Linux 3.4) This subdirectory contains files with predefined events. The contents are strings describing the event settings expressed in terms of the fields found in the previously mentioned./format/ directory. These are not necessarily complete lists of all events supported by a PMU, but usually a subset of events deemed useful or interesting. The content of each file is a list of attribute names separated by commas. Each entry has an optional value (either hex or decimal). If no value is specified, then it is assumed to be a single-bit field with a value of 1. An example entry may look like this:event=0x2,inv,ldlat=3./sys/bus/event_source/devices/*/uevent This file is the standard kernel device interface for injecting hotplug events./sys/bus/event_source/devices/*/cpumask (since Linux 3.7) Thecpumask file contains a comma-separated list of integers that indicate a representative CPU number for each socket (package) on the motherboard. This is needed when setting up uncore or northbridge events, as those PMUs present socket-wide events.On success,perf_event_open() returns the new file descriptor. On error, -1 is returned anderrno is set to indicate the error.
The errors returned byperf_event_open() can be inconsistent, and may vary across processor architectures and performance monitoring units.E2BIGReturned if theperf_event_attr size value is too small (smaller thanPERF_ATTR_SIZE_VER0), too big (larger than the page size), or larger than the kernel supports and the extra bytes are not zero. WhenE2BIGis returned, theperf_event_attr size field is overwritten by the kernel to be the size of the structure it was expecting.EACCESReturned when the requested event requiresCAP_PERFMON (since Linux 5.8) orCAP_SYS_ADMINpermissions (or a more permissive perf_event paranoid setting). Some common cases where an unprivileged process may encounter this error: attaching to a process owned by a different user; monitoring all processes on a given CPU (i.e., specifying thepid argument as -1); and not settingexclude_kernel when the paranoid setting requires it.EBADFReturned if thegroup_fd file descriptor is not valid, or, ifPERF_FLAG_PID_CGROUPis set, the cgroup file descriptor inpid is not valid.EBUSY(since Linux 4.1) Returned if another event already has exclusive access to the PMU.EFAULTReturned if theattr pointer points at an invalid memory address.EINTRReturned when trying to mix perf and ftrace handling for a uprobe.EINVALReturned if the specified event is invalid. There are many possible reasons for this. A not-exhaustive list:sample_freq is higher than the maximum setting; thecpu to monitor does not exist;read_format is out of range;sample_type is out of range; theflags value is out of range;exclusive orpinned set and the event is not a group leader; the eventconfig values are out of range or set reserved bits; the generic event selected is not supported; or there is not enough room to add the selected event.EMFILEEach opened event uses one file descriptor. If a large number of events are opened, the per-process limit on the number of open file descriptors will be reached, and no more events can be created.ENODEVReturned when the event involves a feature not supported by the current CPU.ENOENTReturned if thetype setting is not valid. This error is also returned for some unsupported generic events.ENOSPCPrior to Linux 3.3, if there was not enough room for the event,ENOSPCwas returned. In Linux 3.3, this was changed toEINVAL.ENOSPCis still returned if you try to add more breakpoint events than supported by the hardware.ENOSYSReturned ifPERF_SAMPLE_STACK_USERis set insample_type and it is not supported by hardware.EOPNOTSUPP Returned if an event requiring a specific hardware feature is requested but there is no hardware support. This includes requesting low-skid events if not supported, branch tracing if it is not available, sampling if no PMU interrupt is available, and branch stacks for software events.EOVERFLOW(since Linux 4.8) Returned ifPERF_SAMPLE_CALLCHAINis requested andsample_max_stack is larger than the maximum specified in/proc/sys/kernel/perf_event_max_stack.EPERMReturned on many (but not all) architectures when an unsupportedexclude_hv,exclude_idle,exclude_user, orexclude_kernel setting is specified. It can also happen, as withEACCES, when the requested event requiresCAP_PERFMON(since Linux 5.8) orCAP_SYS_ADMINpermissions (or a more permissive perf_event paranoid setting). This includes setting a breakpoint on a kernel address, and (since Linux 3.13) setting a kernel function-trace tracepoint.ESRCHReturned if attempting to attach to a process that does not exist.
Linux.
perf_event_open() was introduced in Linux 2.6.31 but was calledperf_counter_open(). It was renamed in Linux 2.6.32.
The official way of knowing ifperf_event_open() support is enabled is checking for the existence of the file/proc/sys/kernel/perf_event_paranoid.CAP_PERFMONcapability (since Linux 5.8) provides secure approach to performance monitoring and observability operations in a system according to the principal of least privilege (POSIX IEEE 1003.1e). Accessing system performance monitoring and observability operations usingCAP_PERFMONrather than the much more powerfulCAP_SYS_ADMINexcludes chances to misuse credentials and makes operations more secure.CAP_SYS_ADMINusage for secure system performance monitoring and observability is discouraged in favor of theCAP_PERFMONcapability.
TheF_SETOWN_EXoption tofcntl(2) is needed to properly get overflow signals in threads. This was introduced in Linux 2.6.32. Prior to Linux 2.6.33 (at least for x86), the kernel did not check if events could be scheduled together until read time. The same happens on all known kernels if the NMI watchdog is enabled. This means to see if a given set of events works you have toperf_event_open(), start, then read before you know for sure you can get valid measurements. Prior to Linux 2.6.34, event constraints were not enforced by the kernel. In that case, some events would silently return "0" if the kernel scheduled them in an improper counter slot. Prior to Linux 2.6.34, there was a bug when multiplexing where the wrong results could be returned. Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the kernel if "inherit" is enabled and many threads are started. Prior to Linux 2.6.35,PERF_FORMAT_GROUPdid not work with attached processes. There is a bug in the kernel code between Linux 2.6.36 and Linux 3.0 that ignores the "watermark" field and acts as if a wakeup_event was chosen if the union has a nonzero value in it. From Linux 2.6.31 to Linux 3.4, thePERF_IOC_FLAG_GROUPioctl argument was broken and would repeatedly operate on the event specified rather than iterating across all sibling events in a group. From Linux 3.4 to Linux 3.11, the mmapcap_usr_rdpmc andcap_usr_time bits mapped to the same location. Code should migrate to the newcap_user_rdpmc andcap_user_time fields instead. Always double-check your results! Various generalized events have had wrong values. For example, retired branches measured the wrong thing on AMD machines until Linux 2.6.35.
The following is a short example that measures the total instruction count of a call toprintf(3). #include <err.h> #include <linux/perf_event.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/ioctl.h> #include <sys/syscall.h> #include <sys/types.h> #include <unistd.h> static long perf_event_open(struct perf_event_attr *hw_event, pid_t pid, int cpu, int group_fd, unsigned long flags) { int ret; ret = syscall(SYS_perf_event_open, hw_event, pid, cpu, group_fd, flags); return ret; } int main(void) { int fd; long long count; struct perf_event_attr pe; memset(&pe, 0, sizeof(pe)); pe.type = PERF_TYPE_HARDWARE; pe.size = sizeof(pe); pe.config = PERF_COUNT_HW_INSTRUCTIONS; pe.disabled = 1; pe.exclude_kernel = 1; pe.exclude_hv = 1; fd = perf_event_open(&pe, 0, -1, -1, 0); if (fd == -1) err(EXIT_FAILURE, "Error opening leader %llx\n", pe.config); if (ioctl(fd, PERF_EVENT_IOC_RESET, 0) == -1) err(EXIT_FAILURE, "PERF_EVENT_IOC_RESET"); if (ioctl(fd, PERF_EVENT_IOC_ENABLE, 0) == -1) err(EXIT_FAILURE, "PERF_EVENT_IOC_ENABLE"); printf("Measuring instruction count for this printf\n"); if (ioctl(fd, PERF_EVENT_IOC_DISABLE, 0) == -1) err(EXIT_FAILURE, "PERF_EVENT_IOC_DISABLE"); if (read(fd, &count, sizeof(count)) != sizeof(count)) err(EXIT_FAILURE, "read"); printf("Used %lld instructions\n", count); if (close(fd) == -1) err(EXIT_FAILURE, "close"); }perf(1),fcntl(2),mmap(2),open(2),prctl(2),read(2)Documentation/admin-guide/perf-security.rst in the kernel source tree
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17perf_event_open(2)Pages that refer to this page:bpf(2), gettid(2), mount_setattr(2), openat2(2), syscalls(2), stapprobes(3stap), proc_pid_fd(5), systemd.exec(5), bpf-helpers(7), capabilities(7), cgroups(7)
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