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sched/nohz: Optimize get_nohz_timer_target()

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On a machine, CPU 0 is used for housekeeping, the other 39 CPUs in the
same socket are in nohz_full mode. We can observe huge time burn in the
loop for seaching nearest busy housekeeper cpu by ftrace.

  2)               |                        get_nohz_timer_target() {
  2)   0.240 us    |                          housekeeping_test_cpu();
  2)   0.458 us    |                          housekeeping_test_cpu();

  ...

  2)   0.292 us    |                          housekeeping_test_cpu();
  2)   0.240 us    |                          housekeeping_test_cpu();
  2)   0.227 us    |                          housekeeping_any_cpu();
  2) + 43.460 us   |                        }

This patch optimizes the searching logic by finding a nearest housekeeper
CPU in the housekeeping cpumask, it can minimize the worst searching time
from ~44us to < 10us in my testing. In addition, the last iterated busy
housekeeper can become a random candidate while current CPU is a better
fallback if it is a housekeeper.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/1578876627-11938-1-git-send-email-wanpengli@tencent.com
This commit is contained in:
Wanpeng Li 2020-01-13 08:50:27 +08:00 committed by Ingo Molnar
parent b562d14064
commit e938b9c941
1 changed files with 12 additions and 7 deletions
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19
kernel/sched/core.c
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@ -552,27 +552,32 @@ void resched_cpu(int cpu)
*/
int get_nohz_timer_target(void)
{
int i, cpu = smp_processor_id();
int i, cpu = smp_processor_id(), default_cpu = -1;
struct sched_domain *sd;
if (!idle_cpu(cpu) && housekeeping_cpu(cpu, HK_FLAG_TIMER))
return cpu;
if (housekeeping_cpu(cpu, HK_FLAG_TIMER)) {
if (!idle_cpu(cpu))
return cpu;
default_cpu = cpu;
}
rcu_read_lock();
for_each_domain(cpu, sd) {
for_each_cpu(i, sched_domain_span(sd)) {
for_each_cpu_and(i, sched_domain_span(sd),
housekeeping_cpumask(HK_FLAG_TIMER)) {
if (cpu == i)
continue;
if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
if (!idle_cpu(i)) {
cpu = i;
goto unlock;
}
}
}
if (!housekeeping_cpu(cpu, HK_FLAG_TIMER))
cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
if (default_cpu == -1)
default_cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
cpu = default_cpu;
unlock:
rcu_read_unlock();
return cpu;