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>

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
index fa3d380ca8c0..321693724ad7 100644
--- 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;