linux-kernel - [RFC patch v3 03/20] sched: Avoid task migration within its preferred LLC

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Message-Id: <284223928844c9ae64de1fa142f8db89775de27b.1750268218.git.tim.c.chen@linux.intel.com>
Date: Wed, 18 Jun 2025 11:27:51 -0700
From: Tim Chen <tim.c.chen@...ux.intel.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: Chen Yu <yu.c.chen@...el.com>,
	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>,
	Tim Chen <tim.c.chen@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Libo Chen <libo.chen@...cle.com>,
	Abel Wu <wuyun.abel@...edance.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Len Brown <len.brown@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [RFC patch v3 03/20] sched: Avoid task migration within its preferred LLC

From: Chen Yu <yu.c.chen@...el.com>

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 89db97f8ef02..567ad2a0cfa2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8813,6 +8813,8 @@ 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) {
-- 
2.32.0