linux-kernel - Re: [PATCH] mmc: core: add WQ_PERCPU to alloc

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Message-ID: <CAPDyKFq46qON9W5akWh7CV5fiGSLHV=K9F_Uj6u_g1g08DPV=g@mail.gmail.com>
Date: Tue, 11 Nov 2025 18:12:51 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Marco Crivellari <marco.crivellari@...e.com>, Adrian Hunter <adrian.hunter@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mmc@...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>
Subject: Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

+ Adrian

On Fri, 7 Nov 2025 at 15:17, Marco Crivellari <marco.crivellari@...e.com> wrote:
>
> Currently if a user enqueues 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 consistency 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 continues the effort to refactor workqueue APIs, which began with
> the introduction of new workqueues and a new alloc_workqueue flag in:
>
> commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
> commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")
>
> This change adds a new WQ_PERCPU flag to explicitly request
> alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.
>
> 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.
>
> Once migration is complete, WQ_UNBOUND can be removed and unbound will
> become the implicit default.
>
> Suggested-by: Tejun Heo <tj@...nel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> ---
>  drivers/mmc/core/block.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/mmc/core/block.c b/drivers/mmc/core/block.c
> index c0ffe0817fd4..6a651ddccf28 100644
> --- a/drivers/mmc/core/block.c
> +++ b/drivers/mmc/core/block.c
> @@ -3275,7 +3275,8 @@ static int mmc_blk_probe(struct mmc_card *card)
>         mmc_fixup_device(card, mmc_blk_fixups);
>
>         card->complete_wq = alloc_workqueue("mmc_complete",
> -                                       WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
> +                                       WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_PERCPU,
> +                                       0);

I guess we prefer to keep the existing behaviour to avoid breaking
anything, before continuing with the refactoring. Although I think it
should be fine to use WQ_UNBOUND here.

Looping in Adrian to get his opinion around this.


>         if (!card->complete_wq) {
>                 pr_err("Failed to create mmc completion workqueue");
>                 return -ENOMEM;
> --
> 2.51.1
>

Kind regards
Uffe