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

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Message-ID: <912c038e-b03d-432d-be24-54c0f90193fd-agordeev@linux.ibm.com>
Date: Mon, 8 Sep 2025 13:44:30 +0200
From: Alexander Gordeev <agordeev@...ux.ibm.com>
To: Marco Crivellari <marco.crivellari@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...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>, Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH 2/2] s390: replace use of system_wq with system_percpu_wq

On Fri, Sep 05, 2025 at 11:08:57AM +0200, Marco Crivellari wrote:

Hi Marco,

> 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.

This paragraph is not exactly correct. You switch from system_wq to
system_percpu_wq - which are two different queues with the same
characteristics:

	system_wq = alloc_workqueue("events", 0, 0);                            
	system_percpu_wq = alloc_workqueue("events", 0, 0);                     

> 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.

I do not see this patch does anything like that.

> 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>
> ---
>  arch/s390/kernel/diag/diag324.c  | 4 ++--
>  arch/s390/kernel/hiperdispatch.c | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/s390/kernel/diag/diag324.c b/arch/s390/kernel/diag/diag324.c
> index 7fa4c0b7eb6c..f0a8b4841fb9 100644
> --- a/arch/s390/kernel/diag/diag324.c
> +++ b/arch/s390/kernel/diag/diag324.c
> @@ -116,7 +116,7 @@ static void pibwork_handler(struct work_struct *work)
>  	mutex_lock(&pibmutex);
>  	timedout = ktime_add_ns(data->expire, PIBWORK_DELAY);
>  	if (ktime_before(ktime_get(), timedout)) {
> -		mod_delayed_work(system_wq, &pibwork, nsecs_to_jiffies(PIBWORK_DELAY));
> +		mod_delayed_work(system_percpu_wq, &pibwork, nsecs_to_jiffies(PIBWORK_DELAY));
>  		goto out;
>  	}
>  	vfree(data->pib);
> @@ -174,7 +174,7 @@ long diag324_pibbuf(unsigned long arg)
>  		pib_update(data);
>  		data->sequence++;
>  		data->expire = ktime_add_ns(ktime_get(), tod_to_ns(data->pib->intv));
> -		mod_delayed_work(system_wq, &pibwork, nsecs_to_jiffies(PIBWORK_DELAY));
> +		mod_delayed_work(system_percpu_wq, &pibwork, nsecs_to_jiffies(PIBWORK_DELAY));
>  		first = false;
>  	}
>  	rc = data->rc;
> 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();
>  	return 1;
>  }
> -- 
> 2.51.0
>