Description:¶

The goal of this feature is to provide a kernel debug logging APIwhere log records can be stored efficiently in memory, where each component(e.g. device drivers) can have one separate debug log.One purpose of this is to inspect the debug logs after a production system crashin order to analyze the reason for the crash.

If the system still runs but only a subcomponent which uses dbf fails,it is possible to look at the debug logs on a live system via the Linuxdebugfs filesystem.

The debug feature may also very useful for kernel and driver development.

Design:¶

Kernel components (e.g. device drivers) can register themselves at the debugfeature with the function calldebug_register().This function initializes adebug log for the caller. For each debug log exists a number of debug areaswhere exactly one is active at one time. Each debug area consists of contiguouspages in memory. In the debug areas there are stored debug entries (log records)which are written by event- and exception-calls.

An event-call writes the specified debug entry to the active debugarea and updates the log pointer for the active area. If the endof the active debug area is reached, a wrap around is done (ring buffer)and the next debug entry will be written at the beginning of the activedebug area.

An exception-call writes the specified debug entry to the log andswitches to the next debug area. This is done in order to be surethat the records which describe the origin of the exception are notoverwritten when a wrap around for the current area occurs.

The debug areas themselves are also ordered in form of a ring buffer.When an exception is thrown in the last debug area, the following debugentries are then written again in the very first area.

There are four versions for the event- and exception-calls: One forlogging raw data, one for text, one for numbers (unsigned int and long),and one for sprintf-like formatted strings.

Each debug entry contains the following data:

• Timestamp

• Cpu-Number of calling task

• Level of debug entry (0...6)

• Return Address to caller

• Flag, if entry is an exception or not

The debug logs can be inspected in a live system through entries inthe debugfs-filesystem. Under the toplevel directory “s390dbf” there isa directory for each registered component, which is named like thecorresponding component. The debugfs normally should be mounted to/sys/kernel/debug therefore the debug feature can be accessed under/sys/kernel/debug/s390dbf.

The content of the directories are files which represent different viewsto the debug log. Each component can decide which views should beused through registering them with the functiondebug_register_view().Predefined views for hex/ascii and sprintf data are provided.It is also possible to define other views. The content ofa view can be inspected simply by reading the corresponding debugfs file.

All debug logs have an actual debug level (range from 0 to 6).The default level is 3. Event and Exception functions have alevelparameter. Only debug entries with a level that is lower or equalthan the actual level are written to the log. This means, whenwriting events, high priority log entries should have a low levelvalue whereas low priority entries should have a high one.The actual debug level can be changed with the help of the debugfs-filesystemthrough writing a number string “x” to thelevel debugfs file which isprovided for every debug log. Debugging can be switched off completelyby using “-” on thelevel debugfs file.

Example:

> echo "-" > /sys/kernel/debug/s390dbf/dasd/level

It is also possible to deactivate the debug feature globally for everydebug log. You can change the behavior using 2 sysctl parameters in/proc/sys/s390dbf:

There are currently 2 possible triggers, which stop the debug featureglobally. The first possibility is to use thedebug_active sysctl. Ifset to 1 the debug feature is running. Ifdebug_active is set to 0 thedebug feature is turned off.

The second trigger which stops the debug feature is a kernel oops.That prevents the debug feature from overwriting debug information thathappened before the oops. After an oops you can reactivate the debug featureby piping 1 to/proc/sys/s390dbf/debug_active. Nevertheless, it’s notsuggested to use an oopsed kernel in a production environment.

If you want to disallow the deactivation of the debug feature, you can usethedebug_stoppable sysctl. If you setdebug_stoppable to 0 the debugfeature cannot be stopped. If the debug feature is already stopped, itwill stay deactivated.

Kernel Interfaces:¶

booldebug_next_entry(file_private_info_t*p_info)¶

Go to the next entry

voiddebug_to_act_entry(file_private_info_t*p_info)¶

Go to the currently active entry

booldebug_prev_entry(file_private_info_t*p_info)¶

Go to the previous entry

booldebug_move_entry(file_private_info_t*p_info,boolreverse)¶

Go to next entry in either the forward or backward direction

ssize_tdebug_dump(debug_info_t*id,structdebug_view*view,char*buf,size_tbuf_size,boolreverse)¶

Get a textual representation of debug info, or as much as fits

debug_info_t*debug_register_mode(constchar*name,intpages_per_area,intnr_areas,intbuf_size,umode_tmode,uid_tuid,gid_tgid)¶

creates and initializes debug area.

debug_info_t*debug_register(constchar*name,intpages_per_area,intnr_areas,intbuf_size)¶

creates and initializes debug area with default file mode.

voiddebug_register_static(debug_info_t*id,intpages_per_area,intnr_areas)¶

registers a static debug area

voiddebug_unregister(debug_info_t*id)¶

give back debug area.

voiddebug_set_level(debug_info_t*id,intnew_level)¶

Sets new actual debug level if new_level is valid.

voiddebug_stop_all(void)¶

stops the debug feature if stopping is allowed.

voiddebug_set_critical(void)¶

event/exception functions try lock instead of spin.

intdebug_register_view(debug_info_t*id,structdebug_view*view)¶

registers new debug view and creates debugfs dir entry

intdebug_unregister_view(debug_info_t*id,structdebug_view*view)¶

unregisters debug view and removes debugfs dir entry

booldebug_level_enabled(debug_info_t*id,intlevel)¶

Returns true if debug events for the specified level would be logged. Otherwise returns false.

debug_entry_t*debug_event(debug_info_t*id,intlevel,void*data,intlength)¶

writes binary debug entry to active debug area (if level <= actual debug level)

debug_entry_t*debug_int_event(debug_info_t*id,intlevel,unsignedinttag)¶

writes unsigned integer debug entry to active debug area (if level <= actual debug level)

debug_entry_t*debug_long_event(debug_info_t*id,intlevel,unsignedlongtag)¶

writes unsigned long debug entry to active debug area (if level <= actual debug level)

debug_entry_t*debug_text_event(debug_info_t*id,intlevel,constchar*txt)¶

writes string debug entry in ascii format to active debug area (if level <= actual debug level)

debug_sprintf_event¶

debug_sprintf_event(_id,_level,_fmt,...)

writes debug entry with format string and varargs (longs) to active debug area (if level $<=$ actual debug level).

debug_entry_t*debug_exception(debug_info_t*id,intlevel,void*data,intlength)¶

writes binary debug entry to active debug area (if level <= actual debug level) and switches to next debug area

debug_entry_t*debug_int_exception(debug_info_t*id,intlevel,unsignedinttag)¶

writes unsigned int debug entry to active debug area (if level <= actual debug level) and switches to next debug area

debug_entry_t*debug_long_exception(debug_info_t*id,intlevel,unsignedlongtag)¶

writes long debug entry to active debug area (if level <= actual debug level) and switches to next debug area

debug_entry_t*debug_text_exception(debug_info_t*id,intlevel,constchar*txt)¶

writes string debug entry in ascii format to active debug area (if level <= actual debug level) and switches to next debug area area

debug_sprintf_exception¶

debug_sprintf_exception(_id,_level,_fmt,...)

writes debug entry with format string and varargs (longs) to active debug area (if level <= actual debug level) and switches to next debug area.

DEFINE_STATIC_DEBUG_INFO¶

DEFINE_STATIC_DEBUG_INFO(var,name,pages,nr_areas,buf_size,view)

Define static debug_info_t

Predefined views:¶

externstructdebug_viewdebug_hex_ascii_view;externstructdebug_viewdebug_sprintf_view;

Examples¶

/* * hex_ascii-view Example */#include<linux/init.h>#include<asm/debug.h>staticdebug_info_t*debug_info;staticintinit(void){/* register 4 debug areas with one page each and 4 byte data field */debug_info=debug_register("test",1,4,4);debug_register_view(debug_info,&debug_hex_ascii_view);debug_text_event(debug_info,4,"one ");debug_int_exception(debug_info,4,4711);debug_event(debug_info,3,&debug_info,4);return0;}staticvoidcleanup(void){debug_unregister(debug_info);}module_init(init);module_exit(cleanup);

/* * sprintf-view Example */#include<linux/init.h>#include<asm/debug.h>staticdebug_info_t*debug_info;staticintinit(void){/* register 4 debug areas with one page each and data field for *//* format string pointer + 2 varargs (= 3 * sizeof(long))       */debug_info=debug_register("test",1,4,sizeof(long)*3);debug_register_view(debug_info,&debug_sprintf_view);debug_sprintf_event(debug_info,2,"first event in %s:%i\n",__FILE__,__LINE__);debug_sprintf_exception(debug_info,1,"pointer to debug info: %p\n",&debug_info);return0;}staticvoidcleanup(void){debug_unregister(debug_info);}module_init(init);module_exit(cleanup);

Debugfs Interface¶

Views to the debug logs can be investigated through reading the correspondingdebugfs-files:

Example:

> ls /sys/kernel/debug/s390dbf/dasdflush  hex_ascii  level pages> cat /sys/kernel/debug/s390dbf/dasd/hex_ascii | sort -k2,2 -s00 00974733272:680099 2 - 02 0006ad7e  07 ea 4a 90 | ....00 00974733272:682210 2 - 02 0006ade6  46 52 45 45 | FREE00 00974733272:682213 2 - 02 0006adf6  07 ea 4a 90 | ....00 00974733272:682281 1 * 02 0006ab08  41 4c 4c 43 | EXCP01 00974733272:682284 2 - 02 0006ab16  45 43 4b 44 | ECKD01 00974733272:682287 2 - 02 0006ab28  00 00 00 04 | ....01 00974733272:682289 2 - 02 0006ab3e  00 00 00 20 | ...01 00974733272:682297 2 - 02 0006ad7e  07 ea 4a 90 | ....01 00974733272:684384 2 - 00 0006ade6  46 52 45 45 | FREE01 00974733272:684388 2 - 00 0006adf6  07 ea 4a 90 | ....

See section about predefined views for explanation of the above output!

Changing the debug level¶

Example:

> cat /sys/kernel/debug/s390dbf/dasd/level3> echo "5" > /sys/kernel/debug/s390dbf/dasd/level> cat /sys/kernel/debug/s390dbf/dasd/level5

Flushing debug areas¶

Debug areas can be flushed with piping the number of the desiredarea (0...n) to the debugfs file “flush”. When using “-” all debug areasare flushed.

Examples:

1. Flush debug area 0:

   > echo "0" > /sys/kernel/debug/s390dbf/dasd/flush

2. Flush all debug areas:

   > echo "-" > /sys/kernel/debug/s390dbf/dasd/flush

Changing the size of debug areas¶

To resize a debug area, write the desired page count to the “pages” file.Existing data is preserved if it fits; otherwise, oldest entries are dropped.

Example:

Define 4 pages for the debug areas of debug feature “dasd”:

> echo "4" > /sys/kernel/debug/s390dbf/dasd/pages

Stopping the debug feature¶

Example:

1. Check if stopping is allowed:

   > cat /proc/sys/s390dbf/debug_stoppable

2. Stop debug feature:

   > echo 0 > /proc/sys/s390dbf/debug_active

crash Interface¶

Thecrash tool since v5.1.0 has a built-in commands390dbf to display all the debug logs or export them to the file system.With this tool it is possibleto investigate the debug logs on a live system and with a memory dump aftera system crash.

Investigating raw memory¶

One last possibility to investigate the debug logs at a livesystem and after a system crash is to look at the raw memoryunder VM or at the Service Element.It is possible to find the anchor of the debug-logs throughthedebug_area_first symbol in the System map. Then one hasto follow the correct pointers of the data-structures definedin debug.h and find the debug-areas in memory.Normally modules which use the debug feature will also havea global variable with the pointer to the debug-logs. Followingthis pointer it will also be possible to find the debug logs inmemory.

For this method it is recommended to use ‘16 * x + 4’ byte (x = 0..n)for the length of the data field indebug_register() inorder to see the debug entries well formatted.

Predefined Views¶

There are two predefined views: hex_ascii and sprintf.The hex_ascii view shows the data field in hex and ascii representation(e.g.45434b44|ECKD).

The sprintf view formats the debug entries in the same way as the sprintffunction would do. The sprintf event/exception functions write to thedebug entry a pointer to the format string (size = sizeof(long))and for each vararg a long value. So e.g. for a debug entry with a formatstring plus two varargs one would need to allocate a (3 * sizeof(long))byte data area in thedebug_register() function.

IMPORTANT:

Using “%s” in sprintf event functions is dangerous. You can onlyuse “%s” in the sprintf event functions, if the memory for the passed stringis available as long as the debug feature exists. The reason behind this isthat due to performance considerations only a pointer to the string is storedin the debug feature. If you log a string that is freed afterwards, you willget an OOPS when inspecting the debug feature, because then the debug featurewill access the already freed memory.

NOTE:

If using the sprintf view do NOT use other event/exception functionsthan the sprintf-event and -exception functions.

The format of the hex_ascii and sprintf view is as follows:

• Number of area

• Timestamp (formatted as seconds and microseconds since 00:00:00 CoordinatedUniversal Time (UTC), January 1, 1970)

• level of debug entry

• Exception flag (* = Exception)

• Cpu-Number of calling task

• Return Address to caller

• data field

A typical line of the hex_ascii view will look like the following (first lineis only for explanation and will not be displayed when ‘cating’ the view):

area  time           level exception cpu caller    data (hex + ascii)--------------------------------------------------------------------------00    00964419409:440690 1 -         00  88023fe

Defining views¶

Views are specified with the ‘debug_view’ structure. There are definedcallback functions which are used for reading and writing the debugfs files:

structdebug_view{charname[DEBUG_MAX_PROCF_LEN];debug_prolog_proc_t*prolog_proc;debug_header_proc_t*header_proc;debug_format_proc_t*format_proc;debug_input_proc_t*input_proc;void*private_data;};

where:

typedefint(debug_header_proc_t)(debug_info_t*id,structdebug_view*view,intarea,debug_entry_t*entry,char*out_buf);typedefint(debug_format_proc_t)(debug_info_t*id,structdebug_view*view,char*out_buf,constchar*in_buf);typedefint(debug_prolog_proc_t)(debug_info_t*id,structdebug_view*view,char*out_buf);typedefint(debug_input_proc_t)(debug_info_t*id,structdebug_view*view,structfile*file,constchar*user_buf,size_tin_buf_size,loff_t*offset);

The “private_data” member can be used as pointer to view specific data.It is not used by the debug feature itself.

The output when reading a debugfs file is structured like this:

"prolog_proc output""header_proc output 1"  "format_proc output 1""header_proc output 2"  "format_proc output 2""header_proc output 3"  "format_proc output 3"...

When a view is read from the debugfs, the Debug Feature calls the‘prolog_proc’ once for writing the prolog.Then ‘header_proc’ and ‘format_proc’ are called for eachexisting debug entry.

The input_proc can be used to implement functionality when it is written tothe view (e.g. like withecho"0">/sys/kernel/debug/s390dbf/dasd/level).

For header_proc there can be used the default functiondebug_dflt_header_fn() which is defined in debug.h.and which produces the same header output as the predefined views.E.g:

00 00964419409:440761 2 - 00 88023ec

In order to see how to use the callback functions check the implementationof the default views!

Example:

#include<asm/debug.h>#define UNKNOWNSTR "data: %08x"constchar*messages[]={"This error...........\n","That error...........\n","Problem..............\n","Something went wrong.\n","Everything ok........\n",NULL};staticintdebug_test_format_fn(debug_info_t*id,structdebug_view*view,char*out_buf,constchar*in_buf){inti,rc=0;if(id->buf_size>=4){intmsg_nr=*((int*)in_buf);if(msg_nr<sizeof(messages)/sizeof(char*)-1)rc+=sprintf(out_buf,"%s",messages[msg_nr]);elserc+=sprintf(out_buf,UNKNOWNSTR,msg_nr);}returnrc;}structdebug_viewdebug_test_view={"myview",/* name of view */NULL,/* no prolog */&debug_dflt_header_fn,/* default header for each entry */&debug_test_format_fn,/* our own format function */NULL,/* no input function */NULL/* no private data */};

debug_info_t*debug_info;inti;...debug_info=debug_register("test",0,4,4);debug_register_view(debug_info,&debug_test_view);for(i=0;i<10;i++)debug_int_event(debug_info,1,i);

> cat /sys/kernel/debug/s390dbf/test/myview00 00964419734:611402 1 - 00 88042ca   This error...........00 00964419734:611405 1 - 00 88042ca   That error...........00 00964419734:611408 1 - 00 88042ca   Problem..............00 00964419734:611411 1 - 00 88042ca   Something went wrong.00 00964419734:611414 1 - 00 88042ca   Everything ok........00 00964419734:611417 1 - 00 88042ca   data: 0000000500 00964419734:611419 1 - 00 88042ca   data: 0000000600 00964419734:611422 1 - 00 88042ca   data: 0000000700 00964419734:611425 1 - 00 88042ca   data: 0000000800 00964419734:611428 1 - 00 88042ca   data: 00000009