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timer stats: speedups

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Make timer-stats have almost zero overhead when enabled in the config but
not used.  (this way distros can enable it more easily)

Also update the documentation about overhead of timer_stats - it was
written for the first version which had a global lock and a linear list
walk based lookup ;-)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Ingo Molnar 2007-06-01 00:47:16 -07:00 committed by Linus Torvalds
parent 9fcc15ec3c
commit c1a834dc70
3 changed files with 14 additions and 5 deletions
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7
Documentation/hrtimer/timer_stats.txt
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@ -2,9 +2,10 @@ 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.
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

7
kernel/time/timer_stats.c
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@ -236,10 +236,15 @@ void timer_stats_update_stats(void *timer, pid_t pid, void *startf,
/*
* It doesnt matter which lock we take:
*/
spinlock_t *lock = &per_cpu(lookup_lock, raw_smp_processor_id());
spinlock_t *lock;
struct entry *entry, input;
unsigned long flags;
if (likely(!active))
return;
lock = &per_cpu(lookup_lock, raw_smp_processor_id());
input.timer = timer;
input.start_func = startf;
input.expire_func = timerf;

5
lib/Kconfig.debug
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@ -126,7 +126,10 @@ config TIMER_STATS
reprogrammed. The statistics can be read from /proc/timer_stats.
The statistics collection is started by writing 1 to /proc/timer_stats,
writing 0 stops it. This feature is useful to collect information
about timer usage patterns in kernel and userspace.
about timer usage patterns in kernel and userspace. This feature
is lightweight if enabled in the kernel config but not activated
(it defaults to deactivated on bootup and will only be activated
if some application like powertop activates it explicitly).
config DEBUG_SLAB
bool "Debug slab memory allocations"