timer stats: speedups

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>

diff --git a/Documentation/hrtimer/timer_stats.txt b/Documentation/hrtimer/timer_stats.txt
index 27f782e3593f2f86047fbc2e74e387710f3a0f9a..22b0814d0ad0927024de028303a4af9cce0afc11 100644 (file)
--- a/Documentation/hrtimer/timer_stats.txt
+++ b/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

diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
index fa3d380ca8c0b1aeb77250743d4cca9e25cdc38c..321693724ad743a74d81c2b3add00f3d4ee21237 100644 (file)
--- a/kernel/time/timer_stats.c
+++ b/kernel/time/timer_stats.c
@@ -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;

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1ba77ca7d1658e18080bad7c714974ad01b6f5ad..da95e10cfd7094e8996ab08bb38c1f1bd5d4d6f9 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -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"