linux-kernel - [PATCH v3 09/21] sched/cache: Count tasks prefering destination LLC in a sched group

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Message-Id: <eb97cee4e41dc9a987a21712cd731d254faee9b2.1770760558.git.tim.c.chen@linux.intel.com>
Date: Tue, 10 Feb 2026 14:18:49 -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>,
	Josh Don <joshdon@...gle.com>,
	Gavin Guo <gavinguo@...lia.com>,
	Qais Yousef <qyousef@...alina.io>,
	Libo Chen <libchen@...estorage.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v3 09/21] sched/cache: Count tasks prefering destination LLC in a sched group

During LLC load balancing, tabulate the number of tasks on each runqueue
that prefer the LLC contains the env->dst_cpu in a sched group.

For example, consider a system with 4 LLC sched groups (LLC0 to LLC3)
balancing towards LLC3. LLC0 has 3 tasks preferring LLC3, LLC1 has
2, and LLC2 has 1. LLC0, having the most tasks preferring LLC3, is
selected as the busiest source to pick tasks from.

Within a source LLC, the total number of tasks preferring a destination
LLC is computed by summing counts across all CPUs in that LLC. For
instance, if LLC0 has CPU0 with 2 tasks and CPU1 with 1 task preferring
LLC3, the total for LLC0 is 3.

These statistics allow the load balancer to choose tasks from source
sched groups that best match their preferred 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:
    v2->v3:
    Rename nr_pref_llc to nr_pref_dst_llc for clarification.

 kernel/sched/fair.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4a98aa866d65..bb93cc046d73 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10566,6 +10566,9 @@ struct sg_lb_stats {
 	unsigned int nr_numa_running;
 	unsigned int nr_preferred_running;
 #endif
+#ifdef CONFIG_SCHED_CACHE
+	unsigned int nr_pref_dst_llc;
+#endif
 };
 
 /*
@@ -11034,6 +11037,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 {
 	int i, nr_running, local_group, sd_flags = env->sd->flags;
 	bool balancing_at_rd = !env->sd->parent;
+#ifdef CONFIG_SCHED_CACHE
+	int dst_llc = llc_id(env->dst_cpu);
+#endif
 
 	memset(sgs, 0, sizeof(*sgs));
 
@@ -11054,6 +11060,15 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 		if (cpu_overutilized(i))
 			*sg_overutilized = 1;
 
+#ifdef CONFIG_SCHED_CACHE
+		if (sched_cache_enabled() && llc_id(i) != dst_llc) {
+			struct sched_domain *sd_tmp = rcu_dereference(rq->sd);
+
+			if (valid_llc_buf(sd_tmp, dst_llc))
+				sgs->nr_pref_dst_llc += sd_tmp->pf[dst_llc];
+		}
+#endif
+
 		/*
 		 * No need to call idle_cpu() if nr_running is not 0
 		 */
-- 
2.32.0