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| -rw-r--r-- | Documentation/trace/events.txt | 159 |
1 files changed, 57 insertions, 102 deletions
diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt index abdee664c0f6..f157d7594ea7 100644 --- a/Documentation/trace/events.txt +++ b/Documentation/trace/events.txt @@ -1,9 +1,10 @@ Event Tracing Documentation written by Theodore Ts'o + Updated by Li Zefan -Introduction -============ +1. Introduction +=============== Tracepoints (see Documentation/trace/tracepoints.txt) can be used without creating custom kernel modules to register probe functions @@ -12,30 +13,37 @@ using the event tracing infrastructure. Not all tracepoints can be traced using the event tracing system; the kernel developer must provide code snippets which define how the tracing information is saved into the tracing buffer, and how the -the tracing information should be printed. +tracing information should be printed. -Using Event Tracing -=================== +2. Using Event Tracing +====================== + +2.1 Via the 'set_event' interface +--------------------------------- The events which are available for tracing can be found in the file -/sys/kernel/debug/tracing/available_events. +/debug/tracing/available_events. To enable a particular event, such as 'sched_wakeup', simply echo it -to /sys/debug/tracing/set_event. For example: +to /debug/tracing/set_event. For example: - # echo sched_wakeup > /sys/kernel/debug/tracing/set_event + # echo sched_wakeup >> /debug/tracing/set_event -[ Note: events can also be enabled/disabled via the 'enabled' toggle - found in the /sys/kernel/tracing/events/ hierarchy of directories. ] +[ Note: '>>' is necessary, otherwise it will firstly disable + all the events. ] To disable an event, echo the event name to the set_event file prefixed with an exclamation point: - # echo '!sched_wakeup' >> /sys/kernel/debug/tracing/set_event + # echo '!sched_wakeup' >> /debug/tracing/set_event + +To disable all events, echo an empty line to the set_event file: + + # echo > /debug/tracing/set_event -To disable events, echo an empty line to the set_event file: +To enable all events, echo '*:*' or '*:' to the set_event file: - # echo > /sys/kernel/debug/tracing/set_event + # echo *:* > /debug/tracing/set_event The events are organized into subsystems, such as ext4, irq, sched, etc., and a full event name looks like this: <subsystem>:<event>. The @@ -44,92 +52,39 @@ file. All of the events in a subsystem can be specified via the syntax "<subsystem>:*"; for example, to enable all irq events, you can use the command: - # echo 'irq:*' > /sys/kernel/debug/tracing/set_event - -Defining an event-enabled tracepoint ------------------------------------- - -A kernel developer which wishes to define an event-enabled tracepoint -must declare the tracepoint using TRACE_EVENT instead of DECLARE_TRACE. -This is done via two header files in include/trace. For example, to -event-enable the jbd2 subsystem, we must create two files, -include/trace/jbd2.h and include/trace/jbd2_event_types.h. The -include/trace/jbd2.h file should be included by kernel source files that -will have a tracepoint inserted, and might look like this: - -#ifndef _TRACE_JBD2_H -#define _TRACE_JBD2_H - -#include <linux/jbd2.h> -#include <linux/tracepoint.h> - -#include <trace/jbd2_event_types.h> - -#endif - -In a file that utilizes a jbd2 tracepoint, this header file would be -included. Note that you still have to use DEFINE_TRACE(). So for -example, if fs/jbd2/commit.c planned to use the jbd2_start_commit -tracepoint, it would have the following near the beginning of the file: - -#include <trace/jbd2.h> - -DEFINE_TRACE(jbd2_start_commit); - -Then in the function that would call the tracepoint, it would call the -tracepoint function. (For more information, please see the tracepoint -documentation in Documentation/trace/tracepoints.txt): - - trace_jbd2_start_commit(journal, commit_transaction); - -The code snippets which allow jbd2_start_commit to be an event-enabled -tracepoint are placed in the file include/trace/jbd2_event_types.h: - -/* use <trace/jbd2.h> instead */ -#ifndef TRACE_EVENT -# error Do not include this file directly. -# error Unless you know what you are doing. -#endif - -#undef TRACE_SYSTEM -#define TRACE_SYSTEM jbd2 - -#include <linux/jbd2.h> - -TRACE_EVENT(jbd2_start_commit, - TP_PROTO(journal_t *journal, transaction_t *commit_transaction), - TP_ARGS(journal, commit_transaction), - TP_STRUCT__entry( - __array( char, devname, BDEVNAME_SIZE+24 ) - __field( int, transaction ) - ), - TP_fast_assign( - memcpy(__entry->devname, journal->j_devname, BDEVNAME_SIZE+24); - __entry->transaction = commit_transaction->t_tid; - ), - TP_printk("dev %s transaction %d", - __entry->devname, __entry->transaction) -); - -The TP_PROTO and TP_ARGS are unchanged from DECLARE_TRACE. The new -arguments to TRACE_EVENT are TP_STRUCT__entry, TP_fast_assign, and -TP_printk. - -TP_STRUCT__entry defines the data structure which will be stored in the -trace buffer. Normally, fields in __entry will be arrays or simple -types. It is possible to place data structures in __entry --- however, -pointers in the data structure can not be trusted, since they will be -accessed sometime later by TP_printk, and if the data structure contains -fields that will not or cannot be used by TP_printk, this will waste -space in the trace buffer. In general, data structures should be -avoided, unless they do only contain non-pointer types and all of the -fields will be used by TP_printk. - -TP_fast_assign defines the code snippet which saves information into the -__entry data structure, using the passed-in arguments defined in -TP_PROTO and TP_ARGS. - -Finally, TP_printk will print the __entry data structure. At the time -when the code snippet defined by TP_printk is executed, it will not have -access to the TP_ARGS arguments; it can only use the information saved -in the __entry data structure. + # echo 'irq:*' > /debug/tracing/set_event + +2.2 Via the 'enable' toggle +--------------------------- + +The events available are also listed in /debug/tracing/events/ hierarchy +of directories. + +To enable event 'sched_wakeup': + + # echo 1 > /debug/tracing/events/sched/sched_wakeup/enable + +To disable it: + + # echo 0 > /debug/tracing/events/sched/sched_wakeup/enable + +To enable all events in sched subsystem: + + # echo 1 > /debug/tracing/events/sched/enable + +To eanble all events: + + # echo 1 > /debug/tracing/events/enable + +When reading one of these enable files, there are four results: + + 0 - all events this file affects are disabled + 1 - all events this file affects are enabled + X - there is a mixture of events enabled and disabled + ? - this file does not affect any event + +3. Defining an event-enabled tracepoint +======================================= + +See The example provided in samples/trace_events + |