linux-kernel - Re: [PATCH 2/2] s390: replace use of system_wq with system_percpu

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Message-ID: <f672519b-d21e-4576-8cb8-989b95c88f97@linux.ibm.com>
Date: Mon, 8 Sep 2025 10:33:34 +0200
From: Mete Durlu <meted@...ux.ibm.com>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: 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>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, Heiko Carstens <hca@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 2/2] s390: replace use of system_wq with system_percpu_wq

On 9/5/25 11:08 AM, 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>

If I get this correctly system_wq will be obsolete and users will get
system_percpu_wq instead, which means local cpu gets to deal with the
delayed work and its timer and it has an affinity to that cpu via per
cpu workqueue. In that case;


> diff --git a/arch/s390/kernel/hiperdispatch.c b/arch/s390/kernel/hiperdispatch.c
> index e7b66d046e8d..85b5508ab62c 100644
> --- a/arch/s390/kernel/hiperdispatch.c
> +++ b/arch/s390/kernel/hiperdispatch.c
> @@ -191,7 +191,7 @@ int hd_enable_hiperdispatch(void)
>   		return 0;
>   	if (hd_online_cores <= hd_entitled_cores)
>   		return 0;
> -	mod_delayed_work(system_wq, &hd_capacity_work, HD_DELAY_INTERVAL * hd_delay_factor);
> +	mod_delayed_work(system_percpu_wq, &hd_capacity_work, HD_DELAY_INTERVAL * hd_delay_factor);
>   	hd_update_capacities();

Hiperdispatch's delayed work wouldn't get a noticeable benefit from
utilizing a per-cpu workqueue. We probably settled on system_wq to
utilize the global work queue at the time. Would system_unbound_wq
make more sense here?

Thanks.