timer: Minimize nohz off overhead
If nohz is disabled on the kernel command line the [hr]timer code
still calls wake_up_nohz_cpu() and tick_nohz_full_cpu(), a pretty
pointless exercise. Cache nohz_active in [hr]timer per cpu bases and
avoid the overhead.
Before:
48.10% hog [.] main
15.25% [kernel] [k] _raw_spin_lock_irqsave
9.76% [kernel] [k] _raw_spin_unlock_irqrestore
6.50% [kernel] [k] mod_timer
6.44% [kernel] [k] lock_timer_base.isra.38
3.87% [kernel] [k] detach_if_pending
3.80% [kernel] [k] del_timer
2.67% [kernel] [k] internal_add_timer
1.33% [kernel] [k] __internal_add_timer
0.73% [kernel] [k] timerfn
0.54% [kernel] [k] wake_up_nohz_cpu
After:
48.73% hog [.] main
15.36% [kernel] [k] _raw_spin_lock_irqsave
9.77% [kernel] [k] _raw_spin_unlock_irqrestore
6.61% [kernel] [k] lock_timer_base.isra.38
6.42% [kernel] [k] mod_timer
3.90% [kernel] [k] detach_if_pending
3.76% [kernel] [k] del_timer
2.41% [kernel] [k] internal_add_timer
1.39% [kernel] [k] __internal_add_timer
0.76% [kernel] [k] timerfn
We probably should have a cached value for nohz full in the per cpu
bases as well to avoid the cpumask check. The base cache line is hot
already, the cpumask not necessarily.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Joonwoo Park <joonwoop@codeaurora.org>
Cc: Wenbo Wang <wenbo.wang@memblaze.com>
Link: http://lkml.kernel.org/r/20150526224512.207378134@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 343142e..520499d 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -86,6 +86,7 @@
unsigned long all_timers;
int cpu;
bool migration_enabled;
+ bool nohz_active;
struct tvec_root tv1;
struct tvec tv2;
struct tvec tv3;
@@ -99,7 +100,7 @@
#if defined(CONFIG_SMP) && defined(CONFIG_NO_HZ_COMMON)
unsigned int sysctl_timer_migration = 1;
-void timers_update_migration(void)
+void timers_update_migration(bool update_nohz)
{
bool on = sysctl_timer_migration && tick_nohz_active;
unsigned int cpu;
@@ -111,6 +112,10 @@
for_each_possible_cpu(cpu) {
per_cpu(tvec_bases.migration_enabled, cpu) = on;
per_cpu(hrtimer_bases.migration_enabled, cpu) = on;
+ if (!update_nohz)
+ continue;
+ per_cpu(tvec_bases.nohz_active, cpu) = true;
+ per_cpu(hrtimer_bases.nohz_active, cpu) = true;
}
}
@@ -124,7 +129,7 @@
mutex_lock(&mutex);
ret = proc_dointvec(table, write, buffer, lenp, ppos);
if (!ret && write)
- timers_update_migration();
+ timers_update_migration(false);
mutex_unlock(&mutex);
return ret;
}
@@ -436,8 +441,11 @@
* require special care against races with idle_cpu(), lets deal
* with that later.
*/
- if (!(timer->flags & TIMER_DEFERRABLE) || tick_nohz_full_cpu(base->cpu))
- wake_up_nohz_cpu(base->cpu);
+ if (base->nohz_active) {
+ if (!(timer->flags & TIMER_DEFERRABLE) ||
+ tick_nohz_full_cpu(base->cpu))
+ wake_up_nohz_cpu(base->cpu);
+ }
}
#ifdef CONFIG_TIMER_STATS