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, 4 insertions, 1 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1ba77ca7d165..da95e10cfd70 100644
--- 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"