Root/Documentation/timers/timer_stats.txt

1timer_stats - timer usage statistics
2------------------------------------
3
4timer_stats is a debugging facility to make the timer (ab)usage in a Linux
5system visible to kernel and userspace developers. If enabled in the config
6but not used it has almost zero runtime overhead, and a relatively small
7data structure overhead. Even if collection is enabled runtime all the
8locking is per-CPU and lookup is hashed.
9
10timer_stats should be used by kernel and userspace developers to verify that
11their code does not make unduly use of timers. This helps to avoid unnecessary
12wakeups, which should be avoided to optimize power consumption.
13
14It can be enabled by CONFIG_TIMER_STATS in the "Kernel hacking" configuration
15section.
16
17timer_stats collects information about the timer events which are fired in a
18Linux system over a sample period:
19
20- the pid of the task(process) which initialized the timer
21- the name of the process which initialized the timer
22- the function where the timer was initialized
23- the callback function which is associated to the timer
24- the number of events (callbacks)
25
26timer_stats adds an entry to /proc: /proc/timer_stats
27
28This entry is used to control the statistics functionality and to read out the
29sampled information.
30
31The timer_stats functionality is inactive on bootup.
32
33To activate a sample period issue:
34# echo 1 >/proc/timer_stats
35
36To stop a sample period issue:
37# echo 0 >/proc/timer_stats
38
39The statistics can be retrieved by:
40# cat /proc/timer_stats
41
42The readout of /proc/timer_stats automatically disables sampling. The sampled
43information is kept until a new sample period is started. This allows multiple
44readouts.
45
46Sample output of /proc/timer_stats:
47
48Timerstats sample period: 3.888770 s
49  12, 0 swapper hrtimer_stop_sched_tick (hrtimer_sched_tick)
50  15, 1 swapper hcd_submit_urb (rh_timer_func)
51   4, 959 kedac schedule_timeout (process_timeout)
52   1, 0 swapper page_writeback_init (wb_timer_fn)
53  28, 0 swapper hrtimer_stop_sched_tick (hrtimer_sched_tick)
54  22, 2948 IRQ 4 tty_flip_buffer_push (delayed_work_timer_fn)
55   3, 3100 bash schedule_timeout (process_timeout)
56   1, 1 swapper queue_delayed_work_on (delayed_work_timer_fn)
57   1, 1 swapper queue_delayed_work_on (delayed_work_timer_fn)
58   1, 1 swapper neigh_table_init_no_netlink (neigh_periodic_timer)
59   1, 2292 ip __netdev_watchdog_up (dev_watchdog)
60   1, 23 events/1 do_cache_clean (delayed_work_timer_fn)
6190 total events, 30.0 events/sec
62
63The first column is the number of events, the second column the pid, the third
64column is the name of the process. The forth column shows the function which
65initialized the timer and in parenthesis the callback function which was
66executed on expiry.
67
68    Thomas, Ingo
69
70Added flag to indicate 'deferrable timer' in /proc/timer_stats. A deferrable
71timer will appear as follows
72  10D, 1 swapper queue_delayed_work_on (delayed_work_timer_fn)
73
74

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