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

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Message-ID: <aM1baMg5MDOQ37lp@localhost.localdomain>
Date: Fri, 19 Sep 2025 15:32:24 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Marco Crivellari <marco.crivellari@...e.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v2 5/5] rcu: WQ_PERCPU added to alloc_workqueue users

Le Thu, Sep 18, 2025 at 03:17:52AM -0700, Paul E. McKenney a écrit :
> 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);

sync_wq could be unbound as it's not dealing with per-cpu data.

Thanks.

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

-- 
Frederic Weisbecker
SUSE Labs