Messages in this thread Patch in this message | | | Date | Wed, 26 Jun 2019 17:45:13 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH] sched/nohz: Optimize get nohz timer target |
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On Wed, Jun 19, 2019 at 08:42:19AM +0800, Wanpeng Li wrote: > Cc Frederic, > On Tue, 18 Jun 2019 at 11:13, Wanpeng Li <kernellwp@gmail.com> wrote: > > > > From: Wanpeng Li <wanpengli@tencent.com> > > > > On a machine, cpu 0 is used for housekeeping, other 39 cpus 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. > > > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Signed-off-by: Wanpeng Li <wanpengli@tencent.com> > > --- > > kernel/sched/core.c | 5 +++-- > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 83bd6bb..db550cf 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -548,11 +548,12 @@ int get_nohz_timer_target(void) > > > > 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; > > }
Nice, but you also need to handle the default case that doesn't make much sense anymore. It hasn't ever been clear anyway. The last iterated buzy housekeeper can become a random candidate while current CPU is a better fallback if it is a housekeeper. Also you're enhancing housekeeping_any_cpu() in another patch so give it a better chance:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 4778c48a7fda..c5229d71540a 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -539,27 +539,32 @@ 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;
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