Commit bec2860a authored Dec 06, 2019 by Srikar Dronamraju Committed by Ingo Molnar Jan 28, 2020

sched/fair: Optimize select_idle_core()



Currently we loop through all threads of a core to evaluate if the core is
idle or not. This is unnecessary. If a thread of a core is not idle, skip
evaluating other threads of a core. Also while clearing the cpumask, bits
of all CPUs of a core can be cleared in one-shot.

Collecting ticks on a Power 9 SMT 8 system around select_idle_core
while running schbench shows us

(units are in ticks, hence lesser is better)
Without patch
    N        Min     Max     Median         Avg      Stddev
x 130        151    1083        284   322.72308   144.41494

With patch
    N        Min     Max     Median         Avg      Stddev   Improvement
x 164         88     610        201   225.79268   106.78943        30.03%

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: https://lkml.kernel.org/r/20191206172422.6578-1-srikar@linux.vnet.ibm.com

parent 918229cd

kernel/sched/fair.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -5787,10 +5787,12 @@ static int select_idle_core(struct task_struct p, struct sched_domain sd, int
		bool idle = true;

		for_each_cpu(cpu, cpu_smt_mask(core)) {
		__cpumask_clear_cpu(cpu, cpus);
		if (!available_idle_cpu(cpu))
		if (!available_idle_cpu(cpu)) {
		idle = false;
		break;
		}
		}
		cpumask_andnot(cpus, cpus, cpu_smt_mask(core));

		if (idle)
		return core;

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