linux-kernel - Re: [PATCH v2 3/3] rcu: WQ_UNBOUND added to sync

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Message-ID: <aNFZ1huhgmMkaIWH@2a01cb069018a81087c6c9b3bf9471d3.ipv6.abo.wanadoo.fr>
Date: Mon, 22 Sep 2025 16:14:46 +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 3/3] rcu: WQ_UNBOUND added to sync_wq workqueue

Le Fri, Sep 19, 2025 at 04:50:39PM +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 change add the WQ_UNBOUND flag to sync_wq, to make explicit this
> workqueue can be unbound and that it does not benefit from per-cpu work.
> 
> 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.
> 
> 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 4f3175df5999..7137723f8f95 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4888,7 +4888,7 @@ void __init rcu_init(void)
>  	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_UNBOUND, 0);
>  	WARN_ON(!sync_wq);
>  
>  	/* Respect if explicitly disabled via a boot parameter. */
> -- 
> 2.51.0
> 

-- 
Frederic Weisbecker
SUSE Labs