[patch 28/54] timer stats: speedups

Chris Wright
2007-06-08T07:46:10Z

[-- Attachment #1: timer-stats-speedups.patch --]
[-- Type: text/plain, Size: 3420 bytes --]

-stable review patch. If anyone has any objections, please let us know.
---------------------

From: Ingo Molnar <mingo@elte.hu>

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>
---
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"

--