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Message-ID: <aNFYwpNHTjXvHUVX@2a01cb069018a81087c6c9b3bf9471d3.ipv6.abo.wanadoo.fr>
Date: Mon, 22 Sep 2025 16:10:10 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Michal Hocko <mhocko@...e.com>, Jens Axboe <axboe@...nel.dk>,
"Paul E . McKenney" <paulmck@...nel.org>,
Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
Joel Fernandes <joel@...lfernandes.org>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH v2 2/3] rcu: WQ_PERCPU added to alloc_workqueue users
Le Fri, Sep 19, 2025 at 04:50:38PM +0200, Marco Crivellari a écrit :
> 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 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>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
> ---
> kernel/rcu/tree.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8eff357b0436..4f3175df5999 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4885,7 +4885,7 @@ 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);
> --
> 2.51.0
>
>
--
Frederic Weisbecker
SUSE Labs
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