linux-kernel - [RFC PATCH 1/3] sched/fair: Embed sched_entity into cfs

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Message-ID: <20250604195846.193159-2-zecheng@google.com>
Date: Wed,  4 Jun 2025 19:58:41 +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 1/3] sched/fair: Embed sched_entity into cfs_rq

To improve data locality and avoid pointer chasing, embed struct
sched_entity within struct cfs_rq. This co-locates the runqueue state
(cfs_rq) and the entity's scheduling state (se).

This patch implements the following:

- Adds a struct sched_entity field to struct cfs_rq.

- Modifies alloc_fair_sched_group() and free_fair_sched_group() to
remove the separate allocation and freeing logic for sched_entity
objects themselves.

- The task_group->se pointer array (struct sched_entity **se) is
retained. The pointers in this array are updated to point to the
corresponding embedded &cfs_rq->se for each CPU.

Signed-off-by: Zecheng Li <zecheng@...gle.com>
---
 kernel/sched/fair.c  | 10 +---------
 kernel/sched/sched.h |  4 ++++
 2 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fb9bf995a47..c2af9896eef4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13343,8 +13343,6 @@ void free_fair_sched_group(struct task_group *tg)
 	for_each_possible_cpu(i) {
 		if (tg->cfs_rq)
 			kfree(tg->cfs_rq[i]);
-		if (tg->se)
-			kfree(tg->se[i]);
 	}
 
 	kfree(tg->cfs_rq);
@@ -13374,11 +13372,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
 		if (!cfs_rq)
 			goto err;
 
-		se = kzalloc_node(sizeof(struct sched_entity_stats),
-				  GFP_KERNEL, cpu_to_node(i));
-		if (!se)
-			goto err_free_rq;
-
+		se = &cfs_rq->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 +13380,6 @@ 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..6e26b7d59c13 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -738,6 +738,10 @@ struct cfs_rq {
 	struct list_head	throttled_csd_list;
 #endif /* CONFIG_CFS_BANDWIDTH */
 #endif /* CONFIG_FAIR_GROUP_SCHED */
+#ifdef CONFIG_FAIR_GROUP_SCHED
+	/* sched_entity on parent runqueue */
+	struct sched_entity	se ____cacheline_aligned;
+#endif
 };
 
 #ifdef CONFIG_SCHED_CLASS_EXT
-- 
2.50.0