* [PATCH] sched/nohz: Optimize get nohz timer target
@ 2019-06-18 3:13 Wanpeng Li
2019-06-19 0:42 ` Wanpeng Li
0 siblings, 1 reply; 3+ messages in thread
From: Wanpeng Li @ 2019-06-18 3:13 UTC (permalink / raw)
To: linux-kernel; +Cc: Ingo Molnar, Peter Zijlstra, Ingo Molnar
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;
}
--
2.7.4
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH] sched/nohz: Optimize get nohz timer target 2019-06-18 3:13 [PATCH] sched/nohz: Optimize get nohz timer target Wanpeng Li @ 2019-06-19 0:42 ` Wanpeng Li 2019-06-26 15:45 ` Frederic Weisbecker 0 siblings, 1 reply; 3+ messages in thread From: Wanpeng Li @ 2019-06-19 0:42 UTC (permalink / raw) To: LKML; +Cc: Ingo Molnar, Peter Zijlstra, Ingo Molnar, Frederic Weisbecker 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; > } > -- > 2.7.4 > ^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] sched/nohz: Optimize get nohz timer target 2019-06-19 0:42 ` Wanpeng Li @ 2019-06-26 15:45 ` Frederic Weisbecker 0 siblings, 0 replies; 3+ messages in thread From: Frederic Weisbecker @ 2019-06-26 15:45 UTC (permalink / raw) To: Wanpeng Li Cc: LKML, Ingo Molnar, Peter Zijlstra, Ingo Molnar, Frederic Weisbecker 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; ^ permalink raw reply related [flat|nested] 3+ messages in thread
end of thread, other threads:[~2019-06-26 15:45 UTC | newest] Thread overview: 3+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2019-06-18 3:13 [PATCH] sched/nohz: Optimize get nohz timer target Wanpeng Li 2019-06-19 0:42 ` Wanpeng Li 2019-06-26 15:45 ` Frederic Weisbecker
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