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Message-ID: <20070601061506.GA29595@elte.hu> Date: Fri, 1 Jun 2007 08:15:06 +0200 From: Ingo Molnar <mingo@...e.hu> To: Andrew Morton <akpm@...ux-foundation.org> Cc: linux-kernel@...r.kernel.org, Arjan van de Ven <arjan@...radead.org>, Thomas Gleixner <tglx@...utronix.de> Subject: [patch] timer stats: speedups Subject: [patch] timer stats: speedups From: Ingo Molnar <mingo@...e.hu> 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@...e.hu> --- Documentation/hrtimer/timer_stats.txt | 7 ++++--- kernel/time/timer_stats.c | 7 ++++++- lib/Kconfig.debug | 5 ++++- 3 files changed, 14 insertions(+), 5 deletions(-) Index: linux/Documentation/hrtimer/timer_stats.txt =================================================================== --- linux.orig/Documentation/hrtimer/timer_stats.txt +++ linux/Documentation/hrtimer/timer_stats.txt @@ -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 Index: linux/kernel/time/timer_stats.c =================================================================== --- linux.orig/kernel/time/timer_stats.c +++ linux/kernel/time/timer_stats.c @@ -232,10 +232,15 @@ void timer_stats_update_stats(void *time /* * 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; Index: linux/lib/Kconfig.debug =================================================================== --- linux.orig/lib/Kconfig.debug +++ linux/lib/Kconfig.debug @@ -143,7 +143,10 @@ 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" - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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