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

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Message-ID: <CAPDyKFp3wBv4Kk5TXjQ49CxUjV1sd3uxreX0mOKpD5Gk=OiJrQ@mail.gmail.com>
Date: Wed, 12 Nov 2025 12:45:28 +0100
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Marco Crivellari <marco.crivellari@...e.com>, 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

On Wed, 12 Nov 2025 at 07:49, Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> On 11/11/2025 19:12, Ulf Hansson wrote:
> > + 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.
>
> Typically the work is being queued from the CPU that received the
> interrupt.  I'd assume, running the work on that CPU, as we do now,
> has some merit.
>

Thanks, I get your point!

So, to me it seems like if we want to explore other options, it would
require us to do more analysis to avoid introducing performance
regressions.

BTW, do we know how other block device drivers are dealing with this?

Kind regards
Uffe