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timer: Minimize nohz off overhead

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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>
This commit is contained in:
Thomas Gleixner 2015-05-26 22:50:35 +00:00
parent bc7a34b8b9
commit 683be13a28
5 changed files with 19 additions and 8 deletions
Download patch file Download diff file Expand all files Collapse all files

16
kernel/time/timer.c
View file

@ -86,6 +86,7 @@ struct tvec_base {
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 @@ static DEFINE_PER_CPU(struct tvec_base, tvec_bases);
#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 @@ void timers_update_migration(void)
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 @@ int timer_migration_handler(struct ctl_table *table, int write,
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 @@ static void internal_add_timer(struct tvec_base *base, struct timer_list *timer)
* 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