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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
This commit is contained in:
Srikar Dronamraju 2019-12-06 22:54:22 +05:30 committed by Ingo Molnar
parent 918229cdd5
commit bec2860a2b
1 changed files with 4 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
kernel/sched/fair.c
View File

@ -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;