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authorIngo Molnar <mingo@elte.hu>2007-06-01 00:47:16 -0700
committerChris Wright <chrisw@sous-sol.org>2007-06-11 11:36:48 -0700
commitcf019da09936867c2eea7bc75cf1b1d07e5827d3 (patch)
parent7434011f268211af4a1e4977f5a1eb2004b1a690 (diff)
[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>
3 files changed, 14 insertions, 5 deletions
diff --git a/Documentation/hrtimer/timer_stats.txt b/Documentation/hrtimer/timer_stats.txt
index 27f782e3593f..22b0814d0ad0 100644
--- 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 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;
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 3f3e7403dcac..c93a50246ae8 100644
--- a/lib/Kconfig.debug
+++ b/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).
bool "Debug slab memory allocations"