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 <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/hrtimer/timer_stats.txt

···
 ------------------------------------
 
 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

kernel/time/timer_stats.c

···
 	/*
 	 * 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;

lib/Kconfig.debug

···
 	  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"