linux-kernel - [PATCH 15/19] sched/fair: Respect LLC preference in task migration and detach

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Message-Id: <d3afcff5622222523c843f5c1b023bfe43f9c67c.1760206683.git.tim.c.chen@linux.intel.com>
Date: Sat, 11 Oct 2025 11:24:52 -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: Tim Chen <tim.c.chen@...ux.intel.com>,
	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>,
	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>,
	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>,
	Libo Chen <libo.chen@...cle.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 15/19] sched/fair: Respect LLC preference in task migration and detach

During the final step of load balancing, can_migrate_task() now
considers a task's LLC preference before moving it out of its
preferred LLC.

Additionally, add checks in detach_tasks() to prevent selecting tasks
that prefer their current LLC.

Co-developed-by: Chen Yu <yu.c.chen@...el.com>
Signed-off-by: Chen Yu <yu.c.chen@...el.com>
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
 kernel/sched/fair.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fafbfedb21d..65ff7c306a2f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9801,8 +9801,8 @@ static enum llc_mig can_migrate_llc(int src_cpu, int dst_cpu,
  * Check if task p can migrate from src_cpu to dst_cpu
  * in terms of cache aware load balance.
  */
-static __maybe_unused enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
-							struct task_struct *p)
+static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
+					 struct task_struct *p)
 {
 	struct mm_struct *mm;
 	bool to_pref;
@@ -9969,6 +9969,12 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
 	if (env->flags & LBF_ACTIVE_LB)
 		return 1;
 
+#ifdef CONFIG_SCHED_CACHE
+	if (sched_cache_enabled() &&
+	    can_migrate_llc_task(env->src_cpu, env->dst_cpu, p) == mig_forbid)
+		return 0;
+#endif
+
 	degrades = migrate_degrades_locality(p, env);
 	if (!degrades)
 		hot = task_hot(p, env);
@@ -10227,6 +10233,20 @@ static int detach_tasks(struct lb_env *env)
 		if (env->imbalance <= 0)
 			break;
 
+#ifdef CONFIG_SCHED_CACHE
+		/*
+		 * Don't detach more tasks if the remaining tasks want
+		 * to stay. We know the remaining tasks all prefer the
+		 * current LLC, because after order_tasks_by_llc(), the
+		 * tasks that prefer the current LLC are at the tail of
+		 * the list. The inhibition of detachment is to avoid too
+		 * many tasks being migrated out of the preferred LLC.
+		 */
+		if (sched_cache_enabled() && detached && p->preferred_llc != -1 &&
+		    llc_id(env->src_cpu) == p->preferred_llc)
+			break;
+#endif
+
 		continue;
 next:
 		if (p->sched_task_hot)
-- 
2.32.0