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

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Message-ID: <a2b31164-2175-4fa5-b003-c0be8301bc9a@paulmck-laptop>
Date: Mon, 22 Sep 2025 07:27:28 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Marco Crivellari <marco.crivellari@...e.com>,
	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>,
	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

On Mon, Sep 22, 2025 at 04:14:46PM +0200, Frederic Weisbecker wrote:
> 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>

Applied, thank you both!

I will push these out on my next rebase.

							Thanx, Paul

> > ---
> >  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