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Subject[PATCH v2] sched/fair: Optimize select_idle_core
Date
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%


Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---
Changelog v1->v2:
Updated patch description with measurements made from a debug patch.

kernel/sched/fair.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69a81a5709ff..b9d628128cfc 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5872,10 +5872,12 @@ 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;
--
2.18.1
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