Messages in this thread Patch in this message | | | From | Wanpeng Li <> | Subject | [PATCH v4 2/2] sched/nohz: Optimize get_nohz_timer_target() | Date | Fri, 28 Jun 2019 16:51:40 +0800 |
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From: Wanpeng Li <wanpengli@tencent.com>
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();
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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.
Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Frederic Weisbecker <frederic@kernel.org> Cc: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Wanpeng Li <wanpengli@tencent.com> --- v1 -> v2: * current CPU is a better fallback if it is a housekeeper
kernel/sched/core.c | 19 ++++++++++++------- 1 file changed, 12 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 102dfcf..04a0f6a 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; -- 1.8.3.1
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