linux-kernel - Re: [PATCH 1/1] smp: replace use of system_wq with system_percpu

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Message-ID: <77b495f8-177d-4977-9d09-1dffc29db269@paulmck-laptop>
Date: Fri, 12 Sep 2025 07:24:39 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...hat.com>,
	Rik van Riel <riel@...riel.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 1/1] smp: replace use of system_wq with system_percpu_wq

On Fri, Sep 05, 2025 at 11:09:53AM +0200, Marco Crivellari wrote:
> 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.
> 
> system_wq is a per-CPU worqueue, yet nothing in its name tells about that
> CPU affinity constraint, which is very often not required by users. Make
> it clear by adding a system_percpu_wq.
> 
> queue_work() / queue_delayed_work() mod_delayed_work() will now use the
> new per-cpu wq: whether the user still stick on the old name a warn will
> be printed along a wq redirect to the new one.
> 
> This patch add the new system_percpu_wq except for mm, fs and net
> subsystem, whom are handled in separated patches.
> 
> The old wq will be kept for a few release cylces.
> 
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/smp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 974f3a3962e8..c3b93476d645 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -1146,7 +1146,7 @@ int smp_call_on_cpu(unsigned int cpu, int (*func)(void *), void *par, bool phys)
>  	if (cpu >= nr_cpu_ids || !cpu_online(cpu))
>  		return -ENXIO;
>  
> -	queue_work_on(cpu, system_wq, &sscs.work);
> +	queue_work_on(cpu, system_percpu_wq, &sscs.work);
>  	wait_for_completion(&sscs.done);
>  	destroy_work_on_stack(&sscs.work);
>  
> -- 
> 2.51.0
>