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NAME |SYNOPSIS |DESCRIPTION |KEYWORDS AS EVENT FIELDS |HISTOGRAMS |RETURN VALUE |CREATE A TOOL |EXAMPLE |FILES |SEE ALSO |AUTHOR |REPORTING BUGS |LICENSE |RESOURCES |COPYING |NOTES |COLOPHON | |
LIBTRACEFS(3) libtracefs ManualLIBTRACEFS(3)tracefs_sql - Create a synthetic event via an SQL statement
#include <tracefs.h> struct tracefs_synth *tracefs_sql(struct tep_handle *tep, const char *name, const char *sql_buffer, char **err);
Synthetic events are dynamically created events that attach two existing events together via one or more matching fields between the two events. It can be used to find the latency between the events, or to simply pass fields of the first event on to the second event to display as one event. The Linux kernel interface to create synthetic events is complex, and there needs to be a better way to create synthetic events that is easy and can be understood via existing technology. If you think of each event as a table, where the fields are the column of the table and each instance of the event as a row, you can understand how SQL can be used to attach two events together and form another event (table). Utilizing the SQLSELECT FROM JOINON[WHERE] syntax, a synthetic event can easily be created from two different events. For simple SQL queries to make a histogram instead of a synthetic event, see HISTOGRAMS below.tracefs_sql() takes in atep handler (Seetep_local_events(3)) that is used to verify the events within thesql_buffer expression. Thename is the name of the synthetic event to create. Iferr points to an address of a string, it will be filled with a detailed message on any type of parsing error, including fields that do not belong to an event, or if the events or fields are not properly compared. The example program below is a fully functional parser where it will create a synthetic event from a SQL syntax passed in via the command line or a file. The SQL format is as follows:SELECT<fields>FROM<start-event>JOIN<end-event>ON <matching-fields>WHERE<filter> Note, although the examples show the SQL commands in uppercase, they are not required to be so. That is, you can use "SELECT" or "select" or "sElEct". For example: SELECT syscalls.sys_enter_read.fd, syscalls.sys_exit_read.ret FROM syscalls.sys_enter_read JOIN syscalls.sys_exit_read ON syscalls.sys_enter_read.common_pid = syscalls.sys_exit_write.common_pid Will create a synthetic event that with the fields: u64 fd; s64 ret; Because the function takes atep handle, and usually all event names are unique, you can leave off the system (group) name of the event, andtracefs_sql() will discover the system for you. That is, the above statement would work with: SELECT sys_enter_read.fd, sys_exit_read.ret FROM sys_enter_read JOIN sys_exit_read ON sys_enter_read.common_pid = sys_exit_write.common_pid TheASkeyword can be used to name the fields as well as to give an alias to the events, such that the above can be simplified even more as: SELECT start.fd, end.ret FROM sys_enter_read AS start JOIN sys_exit_read AS end ON start.common_pid = end.common_pid The above aliasessys_enter_read asstartandsys_exit_read asend and uses those aliases to reference the event throughout the statement. Using theASkeyword in the selection portion of the SQL statement will define what those fields will be called in the synthetic event. SELECT start.fd AS filed, end.ret AS return FROM sys_enter_read AS start JOIN sys_exit_read AS end ON start.common_pid = end.common_pid The above labels thefd ofstart asfiledand theret ofend asreturnwhere the synthetic event that is created will now have the fields: u64 filed; s64 return; The fields can also be calculated with results passed to the synthetic event: select start.truesize, end.len, (start.truesize - end.len) as diff from napi_gro_receive_entry as start JOIN netif_receive_skb as end ON start.skbaddr = end.skbaddr Which would show thetruesizeof thenapi_gro_receive_entry event, the actuallen of the content, shown by thenetif_receive_skb, and the delta between the two and expressed by the fielddiff. The code also supports recording the timestamps at either event, and performing calculations on them. For wakeup latency, you have: select start.pid, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) as lat from sched_waking as start JOIN sched_switch as end ON start.pid = end.next_pid The above will create a synthetic event that records thepid of the task being woken up, and the time difference between thesched_waking event and thesched_switch event. TheTIMESTAMP_USECS will truncate the time down to microseconds as the timestamp usually recorded in the tracing buffer has nanosecond resolution. If you do not want that truncation, useTIMESTAMPinstead ofTIMESTAMP_USECS. Because it is so common to have: (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) The above can be represented withTIMESTAMP_DELTA_USECSor if nanoseconds are OK, you can useTIMESTAMP_DELTA. That is, the previous select can also be represented by: select start.pid, TIMESTAMP_DELTA_USECS as lat from sched_waking as start JOIN sched_switch as end ON start.pid = end.next_pid Finally, theWHEREclause can be added, that will let you add filters on either or both events. select start.pid, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) as lat from sched_waking as start JOIN sched_switch as end ON start.pid = end.next_pid WHERE start.prio < 100 && (!(end.prev_pid < 1 || end.prev_prio > 100) || end.prev_pid == 0)NOTE Although both events can be used together in theWHEREclause, they must not be mixed outside the top most "&&" statements. You can not OR (||) the events together, where a filter of one event is OR’d to a filter of the other event. This does not make sense, as the synthetic event requires both events to take place to be recorded. If one is filtered out, then the synthetic event does not execute. select start.pid, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) as lat from sched_waking as start JOIN sched_switch as end ON start.pid = end.next_pid WHERE start.prio < 100 && end.prev_prio < 100 The above is valid. Where as the below is not. select start.pid, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) as lat from sched_waking as start JOIN sched_switch as end ON start.pid = end.next_pid WHERE start.prio < 100 || end.prev_prio < 100 If the kernel supports it, you can pass around a stacktrace between events. select start.prev_pid as pid, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) as delta, start.STACKTRACE as stack FROM sched_switch as start JOIN sched_switch as end ON start.prev_pid = end.next_pid WHERE start.prev_state == 2 The above will record a stacktrace when a task is in the UNINTERRUPTIBLE (blocked) state, and trigger the synthetic event when it is scheduled back in, recording the time delta that it was blocked for. It will record the stacktrace of where it was when it scheduled out along with the delta.
In some cases, an event may have a keyword. For example, regcache_drop_region has "from" as a field and the following will not work select from from regcache_drop_region In such cases, add a backslash to the conflicting field, and this will tell the parser that the "from" is a field and not a keyword: select \from from regcache_drop_region
Simple SQL statements without theJOIN ONmay also be used, which will create a histogram instead. When doing this, the struct tracefs_hist descriptor can be retrieved from the returned synthetic event descriptor via thetracefs_synth_get_start_hist(3). In order to utilize the histogram types (see xxx) the CAST command of SQL can be used. That is: select CAST(common_pid AS comm), CAST(id AS syscall) FROM sys_enter Which produces: # echo 'hist:keys=common_pid.execname,id.syscall' > events/raw_syscalls/sys_enter/trigger # cat events/raw_syscalls/sys_enter/hist { common_pid: bash [ 18248], id: sys_setpgid [109] } hitcount: 1 { common_pid: sendmail [ 1812], id: sys_read [ 0] } hitcount: 1 { common_pid: bash [ 18247], id: sys_getpid [ 39] } hitcount: 1 { common_pid: bash [ 18247], id: sys_dup2 [ 33] } hitcount: 1 { common_pid: gmain [ 13684], id: sys_inotify_add_watch [254] } hitcount: 1 { common_pid: cat [ 18247], id: sys_access [ 21] } hitcount: 1 { common_pid: bash [ 18248], id: sys_getpid [ 39] } hitcount: 1 { common_pid: cat [ 18247], id: sys_fadvise64 [221] } hitcount: 1 { common_pid: sendmail [ 1812], id: sys_openat [257] } hitcount: 1 { common_pid: less [ 18248], id: sys_munmap [ 11] } hitcount: 1 { common_pid: sendmail [ 1812], id: sys_close [ 3] } hitcount: 1 { common_pid: gmain [ 1534], id: sys_poll [ 7] } hitcount: 1 { common_pid: bash [ 18247], id: sys_execve [ 59] } hitcount: 1 Note, string fields may not be cast. The possible types to cast to are:HEX- convert the value to use hex and not decimalSYM- convert a pointer to symbolic (kallsyms values)SYM-OFFSET- convert a pointer to symbolic and include the offset.SYSCALL- convert the number to the mapped system call nameEXECNAMEorCOMM- can only be used with the common_pid field. Will show the task name of the process.LOGorLOG2- bucket the key values in a log 2 values (1, 2, 3-4, 5-8, 9-16, 17-32, ...) The above fields are not case sensitive, and "LOG2" works as good as "log". A special CAST toCOUNTER orCOUNTER will make the field a value and not a key. For example: SELECT common_pid, CAST(bytes_req AS _COUNTER_) FROM kmalloc Which will create echo 'hist:keys=common_pid:vals=bytes_req' > events/kmem/kmalloc/trigger cat events/kmem/kmalloc/hist { common_pid: 1812 } hitcount: 1 bytes_req: 32 { common_pid: 9111 } hitcount: 2 bytes_req: 272 { common_pid: 1768 } hitcount: 3 bytes_req: 1112 { common_pid: 0 } hitcount: 4 bytes_req: 512 { common_pid: 18297 } hitcount: 11 bytes_req: 2004Returns 0 on success and -1 on failure. On failure, iferr is defined, it will be allocated to hold a detailed description of what went wrong if it the error was caused by a parsing error, or that an event, field does not exist or is not compatible with what it was combined with.
The below example is a functional program that can be used to parse SQL commands into synthetic events. man tracefs_sql | sed -ne '/^EXAMPLE/,/FILES/ { /EXAMPLE/d ; /FILES/d ; p}' > sqlhist.c gcc -o sqlhist sqlhist.c `pkg-config --cflags --libs libtracefs` Then you can run the above examples: sudo ./sqlhist 'select start.pid, (end.TIMESTAMP_USECS - start.TIMESTAMP_USECS) as lat from sched_waking as start JOIN sched_switch as end ON start.pid = end.next_pid WHERE start.prio < 100 || end.prev_prio < 100' #include <stdio.h> #include <stdlib.h> #include <stdarg.h> #include <string.h> #include <errno.h> #include <unistd.h> #include <tracefs.h> static void usage(char **argv) { fprintf(stderr, "usage: %s [-ed][-n name][-s][-S fields][-m var][-c var][-T][-t dir][-f file | sql-command-line]\n" " -n name - name of synthetic event 'Anonymous' if left off\n" " -t dir - use dir instead of /sys/kernel/tracing\n" " -e - execute the commands to create the synthetic event\n" " -m - trigger the action when var is a new max.\n" " -c - trigger the action when var changes.\n" " -s - used with -m or -c to do a snapshot of the tracing buffer\n" " -S - used with -m or -c to save fields of the end event (comma deliminated)\n" " -T - used with -m or -c to do both a snapshot and a trace\n" " -f file - read sql lines from file otherwise from the command line\n" " if file is '-' then read from standard input.\n", argv[0]); exit(-1); } enum action { ACTION_DEFAULT = 0, ACTION_SNAPSHOT = (1 << 0), ACTION_TRACE = (1 << 1), ACTION_SAVE = (1 << 2), ACTION_MAX = (1 << 3), ACTION_CHANGE = (1 << 4), }; #define ACTIONS ((ACTION_MAX - 1)) static int do_sql(const char *instance_name, const char *buffer, const char *name, const char *var, const char *trace_dir, bool execute, int action, char **save_fields) { struct tracefs_synth *synth; struct tep_handle *tep; struct trace_seq seq; enum tracefs_synth_handler handler; char *err; int ret; if ((action & ACTIONS) && !var) { fprintf(stderr, "Error: -s, -S and -T not supported without -m or -c"); exit(-1); } if (!name) name = "Anonymous"; trace_seq_init(&seq); tep = tracefs_local_events(trace_dir); if (!tep) { if (!trace_dir) trace_dir = "tracefs directory"; perror(trace_dir); exit(-1); } synth = tracefs_sql(tep, name, buffer, &err); if (!synth) { perror("Failed creating synthetic event!"); if (err) fprintf(stderr, "%s", err); free(err); exit(-1); } if (tracefs_synth_complete(synth)) { if (var) { if (action & ACTION_MAX) handler = TRACEFS_SYNTH_HANDLE_MAX; else handler = TRACEFS_SYNTH_HANDLE_CHANGE; if (action & ACTION_SAVE) { ret = tracefs_synth_save(synth, handler, var, save_fields); if (ret < 0) { err = "adding save"; goto failed_action; } } if (action & ACTION_TRACE) { /* * By doing the trace before snapshot, it will be included * in the snapshot. */ ret = tracefs_synth_trace(synth, handler, var); if (ret < 0) { err = "adding trace"; goto failed_action; } } if (action & ACTION_SNAPSHOT) { ret = tracefs_synth_snapshot(synth, handler, var); if (ret < 0) { err = "adding snapshot"; failed_action: perror(err); if (errno == ENODEV) fprintf(stderr, "ERROR: '%s' is not a variable\n", var); exit(-1); } } } tracefs_synth_echo_cmd(&seq, synth); if (execute) { ret = tracefs_synth_create(synth); if (ret < 0) { fprintf(stderr, "%s\n", tracefs_error_last(NULL)); exit(-1); } } } else { struct tracefs_instance *instance = NULL; struct tracefs_hist *hist; hist = tracefs_synth_get_start_hist(synth); if (!hist) { perror("get_start_hist"); exit(-1); } if (instance_name) { if (execute) instance = tracefs_instance_create(instance_name); else instance = tracefs_instance_alloc(trace_dir, instance_name); if (!instance) { perror("Failed to create instance"); exit(-1); } } tracefs_hist_echo_cmd(&seq, instance, hist, 0); if (execute) { ret = tracefs_hist_start(instance, hist); if (ret < 0) { fprintf(stderr, "%s\n", tracefs_error_last(instance)); exit(-1); } } } tracefs_synth_free(synth); trace_seq_do_printf(&seq); trace_seq_destroy(&seq); return 0; } int main (int argc, char **argv) { char *trace_dir = NULL; char *buffer = NULL; char buf[BUFSIZ]; int buffer_size = 0; const char *file = NULL; const char *instance = NULL; bool execute = false; char **save_fields = NULL; const char *name; const char *var; int action = 0; char *tok; FILE *fp; size_t r; int c; int i; for (;;) { c = getopt(argc, argv, "ht:f:en:m:c:sS:TB:"); if (c == -1) break; switch(c) { case 'h': usage(argv); case 't': trace_dir = optarg; break; case 'f': file = optarg; break; case 'e': execute = true; break; case 'm': action |= ACTION_MAX; var = optarg; break; case 'c': action |= ACTION_CHANGE; var = optarg; break; case 's': action |= ACTION_SNAPSHOT; break; case 'S': action |= ACTION_SAVE; tok = strtok(optarg, ","); while (tok) { save_fields = tracefs_list_add(save_fields, tok); tok = strtok(NULL, ","); } if (!save_fields) { perror(optarg); exit(-1); } break; case 'T': action |= ACTION_TRACE | ACTION_SNAPSHOT; break; case 'B': instance = optarg; break; case 'n': name = optarg; break; } } if ((action & (ACTION_MAX|ACTION_CHANGE)) == (ACTION_MAX|ACTION_CHANGE)) { fprintf(stderr, "Can not use both -m and -c together\n"); exit(-1); } if (file) { if (!strcmp(file, "-")) fp = stdin; else fp = fopen(file, "r"); if (!fp) { perror(file); exit(-1); } while ((r = fread(buf, 1, BUFSIZ, fp)) > 0) { buffer = realloc(buffer, buffer_size + r + 1); strncpy(buffer + buffer_size, buf, r); buffer_size += r; } fclose(fp); if (buffer_size) buffer[buffer_size] = '\0'; } else if (argc == optind) { usage(argv); } else { for (i = optind; i < argc; i++) { r = strlen(argv[i]); buffer = realloc(buffer, buffer_size + r + 2); if (i != optind) buffer[buffer_size++] = ' '; strcpy(buffer + buffer_size, argv[i]); buffer_size += r; } } do_sql(instance, buffer, name, var, trace_dir, execute, action, save_fields); free(buffer); return 0; }tracefs.h Header file to include in order to have access to the library APIs.-ltracefs Linker switch to add when building a program that uses the library.
sqlhist(1),libtracefs(3),libtraceevent(3),trace-cmd(1),tracefs_synth_init(3),tracefs_synth_add_match_field(3),tracefs_synth_add_compare_field(3),tracefs_synth_add_start_field(3),tracefs_synth_add_end_field(3),tracefs_synth_append_start_filter(3),tracefs_synth_append_end_filter(3),tracefs_synth_create(3),tracefs_synth_destroy(3),tracefs_synth_free(3),tracefs_synth_echo_cmd(3),tracefs_hist_alloc(3),tracefs_hist_alloc_2d(3),tracefs_hist_alloc_nd(3),tracefs_hist_free(3),tracefs_hist_add_key(3),tracefs_hist_add_value(3),tracefs_hist_add_name(3),tracefs_hist_start(3),tracefs_hist_destory(3),tracefs_hist_add_sort_key(3),tracefs_hist_sort_key_direction(3)
Steven Rostedt<rostedt@goodmis.org[1]>Tzvetomir Stoyanov<tz.stoyanov@gmail.com[2]>sameeruddin shaik<sameeruddin.shaik8@gmail.com[3]>
Report bugs to <linux-trace-devel@vger.kernel.org[4]>
libtracefs is Free Software licensed under the GNU LGPL 2.1
https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git/
Copyright (C) 2020 VMware, Inc. Free use of this software is granted under the terms of the GNU Public License (GPL).
1. rostedt@goodmis.org mailto:rostedt@goodmis.org 2. tz.stoyanov@gmail.com mailto:tz.stoyanov@gmail.com 3. sameeruddin.shaik8@gmail.com mailto:sameeruddin.shaik8@gmail.com 4. linux-trace-devel@vger.kernel.org mailto:linux-trace-devel@vger.kernel.org
This page is part of thelibtracefs (Linux kernel trace file system library) project. Information about the project can be found at ⟨https://www.trace-cmd.org/⟩. If you have a bug report for this manual page, see ⟨https://www.trace-cmd.org/⟩. This page was obtained from the project's upstream Git repository ⟨https://git.kernel.org/pub/scm/libs/libtrace/libtracefs.git⟩ on 2025-08-11. (At that time, the date of the most recent commit that was found in the repository was 2025-06-02.) 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.orglibtracefs 1.8.1 01/02/2025LIBTRACEFS(3)Pages that refer to this page:sqlhist(1), tracefs_synth_create(3), tracefs_synth_echo_cmd(3)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |