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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /Documentation/timers/timer_stats.txt
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+timer_stats - timer usage statistics
+------------------------------------
+
+timer_stats is a debugging facility to make the timer (ab)usage in a Linux
+system visible to kernel and userspace developers. If enabled in the config
+but not used it has almost zero runtime overhead, and a relatively small
+data structure overhead. Even if collection is enabled runtime all the
+locking is per-CPU and lookup is hashed.
+
+timer_stats should be used by kernel and userspace developers to verify that
+their code does not make unduly use of timers. This helps to avoid unnecessary
+wakeups, which should be avoided to optimize power consumption.
+
+It can be enabled by CONFIG_TIMER_STATS in the "Kernel hacking" configuration
+section.
+
+timer_stats collects information about the timer events which are fired in a
+Linux system over a sample period:
+
+- the pid of the task(process) which initialized the timer
+- the name of the process which initialized the timer
+- the function where the timer was initialized
+- the callback function which is associated to the timer
+- the number of events (callbacks)
+
+timer_stats adds an entry to /proc: /proc/timer_stats
+
+This entry is used to control the statistics functionality and to read out the
+sampled information.
+
+The timer_stats functionality is inactive on bootup.
+
+To activate a sample period issue:
+# echo 1 >/proc/timer_stats
+
+To stop a sample period issue:
+# echo 0 >/proc/timer_stats
+
+The statistics can be retrieved by:
+# cat /proc/timer_stats
+
+While sampling is enabled, each readout from /proc/timer_stats will see
+newly updated statistics. Once sampling is disabled, the sampled information
+is kept until a new sample period is started. This allows multiple readouts.
+
+Sample output of /proc/timer_stats:
+
+Timerstats sample period: 3.888770 s
+ 12, 0 swapper hrtimer_stop_sched_tick (hrtimer_sched_tick)
+ 15, 1 swapper hcd_submit_urb (rh_timer_func)
+ 4, 959 kedac schedule_timeout (process_timeout)
+ 1, 0 swapper page_writeback_init (wb_timer_fn)
+ 28, 0 swapper hrtimer_stop_sched_tick (hrtimer_sched_tick)
+ 22, 2948 IRQ 4 tty_flip_buffer_push (delayed_work_timer_fn)
+ 3, 3100 bash schedule_timeout (process_timeout)
+ 1, 1 swapper queue_delayed_work_on (delayed_work_timer_fn)
+ 1, 1 swapper queue_delayed_work_on (delayed_work_timer_fn)
+ 1, 1 swapper neigh_table_init_no_netlink (neigh_periodic_timer)
+ 1, 2292 ip __netdev_watchdog_up (dev_watchdog)
+ 1, 23 events/1 do_cache_clean (delayed_work_timer_fn)
+90 total events, 30.0 events/sec
+
+The first column is the number of events, the second column the pid, the third
+column is the name of the process. The forth column shows the function which
+initialized the timer and in parenthesis the callback function which was
+executed on expiry.
+
+ Thomas, Ingo
+
+Added flag to indicate 'deferrable timer' in /proc/timer_stats. A deferrable
+timer will appear as follows
+ 10D, 1 swapper queue_delayed_work_on (delayed_work_timer_fn)
+