[PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Marco Crivellari]

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@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.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);
 	if (!card->complete_wq) {
 		pr_err("Failed to create mmc completion workqueue");
 		return -ENOMEM;
-- 
2.51.1

Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Ulf Hansson]

+ Adrian

On Fri, 7 Nov 2025 at 15:17, Marco Crivellari <marco.crivellari@suse.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@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.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

Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Adrian Hunter]

On 11/11/2025 19:12, Ulf Hansson wrote:
> + Adrian
> 
> On Fri, 7 Nov 2025 at 15:17, Marco Crivellari <marco.crivellari@suse.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@kernel.org>
>> Signed-off-by: Marco Crivellari <marco.crivellari@suse.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.

Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Ulf Hansson]

On Wed, 12 Nov 2025 at 07:49, Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 11/11/2025 19:12, Ulf Hansson wrote:
> > + Adrian
> >
> > On Fri, 7 Nov 2025 at 15:17, Marco Crivellari <marco.crivellari@suse.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@kernel.org>
> >> Signed-off-by: Marco Crivellari <marco.crivellari@suse.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

Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Ulf Hansson]

On Fri, 7 Nov 2025 at 15:17, Marco Crivellari <marco.crivellari@suse.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@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>

Applied for next, thanks!

Kind regards
Uffe


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

Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Marco Crivellari]

On Wed, Nov 12, 2025 at 12:48 PM Ulf Hansson <ulf.hansson@linaro.org> wrote:
> [...]
>
> Applied for next, thanks!

Many thanks!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product

Re: [PATCH] mmc: core: add WQ_PERCPU to alloc_workqueue users

[Adrian Hunter]

On 12/11/2025 13:45, Ulf Hansson wrote:
> On Wed, 12 Nov 2025 at 07:49, Adrian Hunter <adrian.hunter@intel.com> wrote:
>>
>> On 11/11/2025 19:12, Ulf Hansson wrote:
>>> + Adrian
>>>
>>> On Fri, 7 Nov 2025 at 15:17, Marco Crivellari <marco.crivellari@suse.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@kernel.org>
>>>> Signed-off-by: Marco Crivellari <marco.crivellari@suse.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?

AFAIK, call blk_mq_complete_request() from the interrupt handler.
mmc_block does that in the case of CQE or HSQ.