[PATCH v2] sched/fair: Clear target from cpus to scan in select_idle_cpu

[Yicong Yang]

Commit 56498cfb045d noticed that "When select_idle_cpu starts scanning for
an idle CPU, it starts with a target CPU that has already been checked
by select_idle_sibling. This patch starts with the next CPU instead."
It only changed the scanning start cpu to target + 1 but still leave
the target in the scanning cpumask. The target still have a chance to be
checked in the last turn. Fix this by clear the target from the cpus
to scan.

Mel Gorman worries bit clear will introduce expensive cost so I
tested hackbench on a 2P Skylake server which shows no obvious
performance degradation.

hackbench-process-pipes
                           5.17-rc1                 patched
Amean     1        0.3332 (   0.00%)      0.3392 *  -1.80%*
Amean     4        0.9536 (   0.00%)      0.9192 *   3.61%*
Amean     7        1.3382 (   0.00%)      1.3226 *   1.17%*
Amean     12       2.1286 (   0.00%)      2.1132 *   0.72%*
Amean     21       3.9210 (   0.00%)      3.8996 *   0.55%*
Amean     30       6.9853 (   0.00%)      6.6562 *   4.71%*
Amean     48      12.5749 (   0.00%)     12.3100 *   2.11%*
Amean     79      17.2422 (   0.00%)     17.1231 *   0.69%*
Amean     110     21.8868 (   0.00%)     21.6772 *   0.96%*
Amean     141     26.9521 (   0.00%)     27.0702 *  -0.44%*
Amean     160     30.5096 (   0.00%)     30.3971 *   0.37%*

Fixes: 56498cfb045d ("sched/fair: Avoid a second scan of target in select_idle_cpu")
Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
---
Change since v1:
- add test in the commit
Link:https://lore.kernel.org/lkml/20211124085401.14411-1-yangyicong@xxxxxxxxxxxxx/

 kernel/sched/fair.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 5146163bfabb..6b8bf86f5eff 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6281,6 +6281,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 		return -1;
 
 	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+	cpumask_clear_cpu(target, cpus);
 
 	if (sched_feat(SIS_PROP) && !has_idle_core) {
 		u64 avg_cost, avg_idle, span_avg;
-- 
2.24.0