Subject: [patch] timer stats: speedups
From: Ingo Molnar <[EMAIL PROTECTED]>
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 <[EMAIL PROTECTED]>
---
Documentation/hrtimer/timer_stats.txt | 7 ++++---
kernel/time/timer_stats.c | 7 ++++++-
lib/Kconfig.debug | 5 ++++-
3 files changed, 14 insertions(+), 5 deletions(-)
Index: linux/Documentation/hrtimer/timer_stats.txt
===================================================================
--- linux.orig/Documentation/hrtimer/timer_stats.txt
+++ linux/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
Index: linux/kernel/time/timer_stats.c
===================================================================
--- linux.orig/kernel/time/timer_stats.c
+++ linux/kernel/time/timer_stats.c
@@ -232,10 +232,15 @@ 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;
Index: linux/lib/Kconfig.debug
===================================================================
--- linux.orig/lib/Kconfig.debug
+++ linux/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"
-
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