linux-kernel - [RFC PATCH v4 03/28] sched: Avoid task migration within its preferred LLC

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Message-Id: <37376d2072f6175d2fb909a29b66a3da0bcfcce3.1754712565.git.tim.c.chen@linux.intel.com>
Date: Sat,  9 Aug 2025 13:01:41 +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 03/28] sched: Avoid task migration within its preferred LLC

It was found that when running schbench, there is a
significant amount of in-LLC task migrations, even if
the wakee is woken up on its preferred LLC. This
leads to core-to-core latency and impairs performance.

Inhibit task migration if the wakee is already in its
preferred LLC.

Signed-off-by: Chen Yu <yu.c.chen@...el.com>
---
 kernel/sched/fair.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e97ab46509e3..00bd0d25bc91 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8835,6 +8835,8 @@ static int select_cache_cpu(struct task_struct *p, int prev_cpu)
 	if (cpu < 0)
 		return prev_cpu;
 
+	if (cpus_share_cache(cpu, prev_cpu))
+		return prev_cpu;
 
 	if (static_branch_likely(&sched_numa_balancing) &&
 	    __migrate_degrades_locality(p, prev_cpu, cpu, false) > 0) {
-- 
2.25.1