-stable review patch. If anyone has any objections, please let us know. --------------------- From: Ingo Molnar 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 ;-) Andrew says: And this. Not a bugfix, but trivial and obvious and apparently some distros don't want to enable timer_stats because of the performance issue, but powertop uses timer_stats. Ingo replies: seconded. I have tested this with and without CONFIG_TIMER_STATS, with and without timer_stats collection activated. Signed-off-by: Ingo Molnar Cc: Thomas Gleixner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds Signed-off-by: Chris Wright Signed-off-by: Greg Kroah-Hartman --- Documentation/hrtimer/timer_stats.txt | 7 ++++--- kernel/time/timer_stats.c | 7 ++++++- lib/Kconfig.debug | 5 ++++- 3 files changed, 14 insertions(+), 5 deletions(-) --- linux-2.6.21.4.orig/Documentation/hrtimer/timer_stats.txt +++ linux-2.6.21.4/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 --- linux-2.6.21.4.orig/kernel/time/timer_stats.c +++ linux-2.6.21.4/kernel/time/timer_stats.c @@ -236,10 +236,15 @@ 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; --- linux-2.6.21.4.orig/lib/Kconfig.debug +++ linux-2.6.21.4/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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/