linux-kernel - [PATCH v2 13/23] sched/cache: Handle moving single tasks to/from their preferred LLC

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Message-Id: <d561c06433e15279674895d3de430a4313dbf973.1764801860.git.tim.c.chen@linux.intel.com>
Date: Wed,  3 Dec 2025 15:07:32 -0800
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>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc: Tim Chen <tim.c.chen@...ux.intel.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>,
	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>,
	Vern Hao <haoxing990@...il.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>,
	Adam Li <adamli@...amperecomputing.com>,
	Aaron Lu <ziqianlu@...edance.com>,
	Tim Chen <tim.c.chen@...el.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 13/23] sched/cache: Handle moving single tasks to/from their preferred LLC

If the busiest runqueue has only one task, active balancing may be
invoked to move it. However, before migration, check whether the task
is running on its preferred LLC.

Do not move a lone task to another LLC if it would move the task
away from its preferred LLC or cause excessive imbalance between LLCs.

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>
---

Notes:
    v1->v2: Remove uneeded preferred LLC migration check from
            active_load_balance_cpu_stop().

 kernel/sched/fair.c | 51 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 529adf342ce0..aed3fab98d7c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9878,12 +9878,57 @@ static __maybe_unused enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu
 			       task_util(p), to_pref);
 }
 
+/*
+ * Check if active load balance breaks LLC locality in
+ * terms of cache aware load balance.
+ */
+static inline bool
+break_llc_locality(struct lb_env *env)
+{
+	if (!sched_cache_enabled())
+		return false;
+
+	if (cpus_share_cache(env->src_cpu, env->dst_cpu))
+		return false;
+	/*
+	 * All tasks prefer to stay on their current CPU.
+	 * Do not pull a task from its preferred CPU if:
+	 * 1. It is the only task running there; OR
+	 * 2. Migrating it away from its preferred LLC would violate
+	 *    the cache-aware scheduling policy.
+	 */
+	if (env->src_rq->nr_pref_llc_running == env->src_rq->cfs.h_nr_runnable) {
+		unsigned long util = 0;
+		struct task_struct *cur;
+
+		if (env->src_rq->nr_running <= 1)
+			return true;
+
+		rcu_read_lock();
+		cur = rcu_dereference(env->src_rq->curr);
+		if (cur)
+			util = task_util(cur);
+		rcu_read_unlock();
+
+		if (can_migrate_llc(env->src_cpu, env->dst_cpu,
+				    util, false) == mig_forbid)
+			return true;
+	}
+
+	return false;
+}
 #else
 static inline bool get_llc_stats(int cpu, unsigned long *util,
 				 unsigned long *cap)
 {
 	return false;
 }
+
+static inline bool
+break_llc_locality(struct lb_env *env)
+{
+	return false;
+}
 #endif
 /*
  * can_migrate_task - may task p from runqueue rq be migrated to this_cpu?
@@ -12279,6 +12324,9 @@ static int need_active_balance(struct lb_env *env)
 {
 	struct sched_domain *sd = env->sd;
 
+	if (break_llc_locality(env))
+		return 0;
+
 	if (asym_active_balance(env))
 		return 1;
 
@@ -12298,7 +12346,8 @@ static int need_active_balance(struct lb_env *env)
 			return 1;
 	}
 
-	if (env->migration_type == migrate_misfit)
+	if (env->migration_type == migrate_misfit ||
+	    env->migration_type == migrate_llc_task)
 		return 1;
 
 	return 0;
-- 
2.32.0