[PATCH v2] sched/fair: Optimize select_idle_core

[Srikar Dronamraju]

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@xxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
---
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