linux-kernel - [PATCH 1/1] sched: replace use of system_unbound_wq with system_dfl

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Message-ID: <20250905090928.108703-2-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:09:28 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org
Cc: Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 1/1] sched: replace use of system_unbound_wq with system_dfl_wq

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the API.

system_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

Adding system_dfl_wq to encourage its use when unbound work should be used.

queue_work() / queue_delayed_work() / mod_delayed_work() will now use the
new unbound wq: whether the user still use the old wq a warn will be
printed along with a wq redirect to the new one.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
---
 kernel/sched/core.c | 4 ++--
 kernel/sched/ext.c  | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c81cf642dba0..baa096e543f1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5772,7 +5772,7 @@ static void sched_tick_remote(struct work_struct *work)
 	os = atomic_fetch_add_unless(&twork->state, -1, TICK_SCHED_REMOTE_RUNNING);
 	WARN_ON_ONCE(os == TICK_SCHED_REMOTE_OFFLINE);
 	if (os == TICK_SCHED_REMOTE_RUNNING)
-		queue_delayed_work(system_unbound_wq, dwork, HZ);
+		queue_delayed_work(system_dfl_wq, dwork, HZ);
 }
 
 static void sched_tick_start(int cpu)
@@ -5791,7 +5791,7 @@ static void sched_tick_start(int cpu)
 	if (os == TICK_SCHED_REMOTE_OFFLINE) {
 		twork->cpu = cpu;
 		INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
-		queue_delayed_work(system_unbound_wq, &twork->work, HZ);
+		queue_delayed_work(system_dfl_wq, &twork->work, HZ);
 	}
 }
 
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 66bcd40a28ca..b1b79143e391 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3514,7 +3514,7 @@ static void scx_watchdog_workfn(struct work_struct *work)
 
 		cond_resched();
 	}
-	queue_delayed_work(system_unbound_wq, to_delayed_work(work),
+	queue_delayed_work(system_dfl_wq, to_delayed_work(work),
 			   scx_watchdog_timeout / 2);
 }
 
@@ -5403,7 +5403,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 
 	WRITE_ONCE(scx_watchdog_timeout, timeout);
 	WRITE_ONCE(scx_watchdog_timestamp, jiffies);
-	queue_delayed_work(system_unbound_wq, &scx_watchdog_work,
+	queue_delayed_work(system_dfl_wq, &scx_watchdog_work,
 			   scx_watchdog_timeout / 2);
 
 	/*
-- 
2.51.0