linux-kernel - [RFC PATCH v4 04/28] sched: Avoid calculating the cpumask if the system is overloaded

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Message-Id: <88d1c3bc1e817cc72346f566153a4618604b9ecd.1754712565.git.tim.c.chen@linux.intel.com>
Date: Sat,  9 Aug 2025 13:02:04 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Libo Chen <libo.chen@...cle.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Jianyong Wu <jianyong.wu@...look.com>,
	Yangyu Chen <cyy@...self.name>,
	Tingyin Duan <tingyin.duan@...il.com>,
	Vern Hao <vernhao@...cent.com>,
	Len Brown <len.brown@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aubrey Li <aubrey.li@...el.com>,
	Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>,
	Chen Yu <yu.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH v4 04/28] sched: Avoid calculating the cpumask if the system is overloaded

From: K Prateek Nayak <kprateek.nayak@....com>

If SIS_UTIL terminates the search for idle CPUs, the result of
cpumask_and() becomes irrelevant. Given that select_idle_cpu()
may now be invoked twice per wake-up within select_idle_sibling()
due to cache-aware wake-ups, this overhead can be observed in
benchmarks such as hackbench.

To conserve additional cycles-particularly in scenarios where
the LLC is frequently targeted and the search aborts because
the LLC is busy - calculate the cpumask only when the system is
not overloaded.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 kernel/sched/fair.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 00bd0d25bc91..a7be5c5ecba3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7940,8 +7940,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 	int i, cpu, idle_cpu = -1, nr = INT_MAX;
 	struct sched_domain_shared *sd_share;
 
-	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
-
 	if (sched_feat(SIS_UTIL)) {
 		sd_share = rcu_dereference(per_cpu(sd_llc_shared, target));
 		if (sd_share) {
@@ -7953,6 +7951,8 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
 		}
 	}
 
+	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+
 	if (static_branch_unlikely(&sched_cluster_active)) {
 		struct sched_group *sg = sd->groups;
 
-- 
2.25.1