[PATCH] 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 ;-)

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 <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>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/time/timer_stats.c b/kernel/time/timer_stats.c
index e2923eda2576..73ddbeb4af3a 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;