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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)

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