linux-kernel - [PATCH v3] sched/fair: do not scan twice in detach

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Message-Id: <20250718063523.9232-1-shijie@os.amperecomputing.com>
Date: Fri, 18 Jul 2025 14:35:23 +0800
From: Huang Shijie <shijie@...amperecomputing.com>
To: mingo@...hat.com,
	peterz@...radead.org,
	juri.lelli@...hat.com,
	vincent.guittot@...aro.org,
	vschneid@...hat.com
Cc: patches@...erecomputing.com,
	cl@...ux.com,
	Shubhang@...amperecomputing.com,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	linux-kernel@...r.kernel.org,
	Huang Shijie <shijie@...amperecomputing.com>
Subject: [PATCH v3] sched/fair: do not scan twice in detach_tasks()

When detach_tasks() scans the src_cpu's task list, the task list
may shrink during the scanning. For example, the task list
may have four tasks at the beginning, it may becomes to two
during the scanning in detach_tasks():
    Task list at beginning : "ABCD"
    Task list in scanning  : "CD"

    (ABCD stands for differnt tasks.)

In this scenario, the env->loop_max is still four, so
detach_tasks() may scan twice for some tasks:
    the scanning order maybe : "DCDC"

In the Specjbb test, this issue can be catched many times.
(Over 330,000 times in a 30-min Specjbb test)

The patch introduces "first_back" to record the first task which
is put back to the task list. If we get a task which is equal to
first_back, we break the loop, and avoid to scan twice for it.

Signed-off-by: Huang Shijie <shijie@...amperecomputing.com>
---
v2 --> v3:
    Fix a typo in the commit message.
    v2: https://lore.kernel.org/all/20250718054709.8781-1-shijie@os.amperecomputing.com/

v1 --> v2:
    Add more comment from Valentin Schneider
    v1: https://lore.kernel.org/all/20250707083636.38380-1-shijie@os.amperecomputing.com/
---
 kernel/sched/fair.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7e2963efe800..7cc9d50e3e11 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9443,6 +9443,7 @@ static int detach_tasks(struct lb_env *env)
 {
 	struct list_head *tasks = &env->src_rq->cfs_tasks;
 	unsigned long util, load;
+	struct task_struct *first_back = NULL;
 	struct task_struct *p;
 	int detached = 0;
 
@@ -9481,6 +9482,12 @@ static int detach_tasks(struct lb_env *env)
 		}
 
 		p = list_last_entry(tasks, struct task_struct, se.group_node);
+		/*
+		 * We're back to an already visited task that couldn't be
+		 * detached, we've seen all there is to see.
+		 */
+		if (p == first_back)
+			break;
 
 		if (!can_migrate_task(p, env))
 			goto next;
@@ -9562,6 +9569,8 @@ static int detach_tasks(struct lb_env *env)
 			schedstat_inc(p->stats.nr_failed_migrations_hot);
 
 		list_move(&p->se.group_node, tasks);
+		if (!first_back)
+			first_back = p;
 	}
 
 	/*
-- 
2.40.1