timer statistics: fix race

[deliverable/linux.git] / Documentation / hrtimer / timer_stats.txt

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. It is not intended for
production usage as it adds significant overhead to the (hr)timer code and the
(hr)timer data structures.

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 intialized
- 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

The readout of /proc/timer_stats automatically disables sampling. 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 parantheses the callback function which was
executed on expiry.

    Thomas, Ingo