linux-kernel - [RFC PATCH v2 1/3] sched/fair: Co-locate cfs_rq and sched

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Message-ID: <20250609193834.2556866-2-zecheng@google.com>
Date: Mon,  9 Jun 2025 19:38:31 +0000
From: Zecheng Li <zecheng@...gle.com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
Cc: 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>, Xu Liu <xliuprof@...gle.com>, 
	Blake Jones <blakejones@...gle.com>, Josh Don <joshdon@...gle.com>, linux-kernel@...r.kernel.org, 
	Zecheng Li <zecheng@...gle.com>
Subject: [RFC PATCH v2 1/3] sched/fair: Co-locate cfs_rq and sched_entity

Improve data locality and reduce pointer chasing by allocating struct
cfs_rq and struct sched_entity together for non-root task groups. This
is achieved by introducing a new combined struct cfs_rq_with_se, that
holds both objects in contiguous memory.

This patch:

- Defines the new struct cfs_rq_with_se.

- Modifies alloc_fair_sched_group() and free_fair_sched_group() to
allocate and free the new struct as a single unit.

- Modifies the per-CPU pointers in task_group->se and task_group->cfs_rq
to point to the members in the new combined structure.

Signed-off-by: Zecheng Li <zecheng@...gle.com>
---
 kernel/sched/fair.c  | 23 ++++++++++-------------
 kernel/sched/sched.h |  8 ++++++++
 2 files changed, 18 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fb9bf995a47..cd090ceec633 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13341,10 +13341,11 @@ void free_fair_sched_group(struct task_group *tg)
 	int i;
 
 	for_each_possible_cpu(i) {
-		if (tg->cfs_rq)
-			kfree(tg->cfs_rq[i]);
-		if (tg->se)
-			kfree(tg->se[i]);
+		if (tg->cfs_rq && tg->cfs_rq[i]) {
+			struct cfs_rq_with_se *combined =
+				container_of(tg->cfs_rq[i], struct cfs_rq_with_se, cfs_rq);
+			kfree(combined);
+		}
 	}
 
 	kfree(tg->cfs_rq);
@@ -13353,6 +13354,7 @@ void free_fair_sched_group(struct task_group *tg)
 
 int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 {
+	struct cfs_rq_with_se *combined;
 	struct sched_entity *se;
 	struct cfs_rq *cfs_rq;
 	int i;
@@ -13369,16 +13371,13 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 	init_cfs_bandwidth(tg_cfs_bandwidth(tg), tg_cfs_bandwidth(parent));
 
 	for_each_possible_cpu(i) {
-		cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
+		combined = kzalloc_node(sizeof(struct cfs_rq_with_se),
 				      GFP_KERNEL, cpu_to_node(i));
-		if (!cfs_rq)
+		if (!combined)
 			goto err;
 
-		se = kzalloc_node(sizeof(struct sched_entity_stats),
-				  GFP_KERNEL, cpu_to_node(i));
-		if (!se)
-			goto err_free_rq;
-
+		cfs_rq = &combined->cfs_rq;
+		se = &combined->se;
 		init_cfs_rq(cfs_rq);
 		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
 		init_entity_runnable_average(se);
@@ -13386,8 +13385,6 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 
 	return 1;
 
-err_free_rq:
-	kfree(cfs_rq);
 err:
 	return 0;
 }
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 47972f34ea70..af23917194fb 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -740,6 +740,14 @@ struct cfs_rq {
 #endif /* CONFIG_FAIR_GROUP_SCHED */
 };
 
+#ifdef CONFIG_FAIR_GROUP_SCHED
+struct cfs_rq_with_se {
+	struct cfs_rq 		cfs_rq;
+	/* cfs_rq's sched_entity on parent runqueue */
+	struct sched_entity 	se  ____cacheline_aligned;
+};
+#endif
+
 #ifdef CONFIG_SCHED_CLASS_EXT
 /* scx_rq->flags, protected by the rq lock */
 enum scx_rq_flags {
-- 
2.50.0.rc0.604.gd4ff7b7c86-goog