linux-kernel - [PATCH v2 5/5] rcu: WQ_PERCPU added to alloc

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Message-Id: <20250918101752.2592512-5-paulmck@kernel.org>
Date: Thu, 18 Sep 2025 03:17:52 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	Marco Crivellari <marco.crivellari@...e.com>,
	Tejun Heo <tj@...nel.org>,
	"Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH v2 5/5] rcu: WQ_PERCPU added to alloc_workqueue users

From: Marco Crivellari <marco.crivellari@...e.com>

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.

alloc_workqueue() treats all queues as per-CPU by default, while unbound
workqueues must opt-in via WQ_UNBOUND.

This default is suboptimal: most workloads benefit from unbound queues,
allowing the scheduler to place worker threads where they’re needed and
reducing noise when CPUs are isolated.

This default is suboptimal: most workloads benefit from unbound queues,
allowing the scheduler to place worker threads where they’re needed and
reducing noise when CPUs are isolated.

This patch adds a new WQ_PERCPU flag to explicitly request the use of
the per-CPU behavior. Both flags coexist for one release cycle to allow
callers to transition their calls.

Once migration is complete, WQ_UNBOUND can be removed and unbound will
become the implicit default.

With the introduction of the WQ_PERCPU flag (equivalent to !WQ_UNBOUND),
any alloc_workqueue() caller that doesn’t explicitly specify WQ_UNBOUND
must now use WQ_PERCPU.

All existing users have been updated accordingly.

Suggested-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1291e0761d70ab..c51c4a0af6aa5e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4890,10 +4890,10 @@ void __init rcu_init(void)
 	rcutree_online_cpu(cpu);

 	/* Create workqueue for Tree SRCU and for expedited GPs. */
-	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
+	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM | WQ_PERCPU, 0);
 	WARN_ON(!rcu_gp_wq);

-	sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
+	sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_PERCPU, 0);
 	WARN_ON(!sync_wq);

 	/* Respect if explicitly disabled via a boot parameter. */
-- 
2.40.1