[PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Marco Crivellari]

Hi!

Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:

        "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
        https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/

=== Current situation: problems ===

Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.

This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
        schedule_delayed_work(, 0);

This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:

        schedule_delayed_work(, 1);

Will move the timer on an housekeeping CPU, and schedule the work there.

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.

=== Plan and future plans ===

This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.

These are the main steps:
1)  API refactoring (that this patch is introducing)
    -   Make more clear and uniform the system wq names, both per-cpu and
        unbound. This to avoid any possible confusion on what should be
        used.

    -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
        introduced in this patchset and used on all the callers that are not
        currently using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a future release cycle.

        Most users don't need to be per-cpu, because they don't have
        locality requirements, because of that, a next future step will be
        make "unbound" the default behavior.

2)  Check who really needs to be per-cpu
    -   Remove the WQ_PERCPU flag when is not strictly required.

3)  Add a new API (prefer local cpu)
    -   There are users that don't require a local execution, like mentioned
        above; despite that, local execution yeld to performance gain.

        This new API will prefer the local execution, without requiring it.

=== Introduced Changes by this series ===

1) [P 1] add WQ_PERCPU to remaining alloc_workqueue() users

        Every alloc_workqueue() caller should use one among WQ_PERCPU or
        WQ_UNBOUND. WQ_PERCPU has been added to every user that is not
		actually using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a next release cycle.

2) [P 2] Replace use of system_wq

        system_wq is a per-CPU workqueue, but his name is not clear.

        Because of that, system_wq has been renamed in system_percpu_wq.

3) [P 3] Replace use of system_wq with system_dfl_wq
			(arch/s390/kernel/hiperdispatch.c)

		 system_wq is a per-cpu wq but hiperdispatch users does not benefit
		 from it. This has been changed with system_dfl_wq (the new unbound wq).

Thanks!

---
Changes in v2:
- New in the series, 2/3: arch/s390/kernel/hiperdispatch.c does not benefit from
  system_wq: this has been converted directly to system_unbound_wq.
- the others system_wq users are converted to system_percpu_wq in patch 3/3


Marco Crivellari (3):
  drivers/s390: WQ_PERCPU added to alloc_workqueue users
  s390/diag324: replace use of system_wq with system_percpu_wq
  s390: replace use of system_wq with system_dfl_wq

 arch/s390/kernel/diag/diag324.c  | 4 ++--
 arch/s390/kernel/hiperdispatch.c | 2 +-
 drivers/s390/char/tape_3590.c    | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

-- 
2.51.0

[PATCH v2 1/3] drivers/s390: WQ_PERCPU added to alloc_workqueue users

[Marco Crivellari]

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.

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 patch adds a new WQ_PERCPU flag to explicitly request the use of
the per-CPU behavior. Both flags coexist for one release cycle to allow
callers to transition their calls.

Once migration is complete, WQ_UNBOUND can be removed and unbound will
become the implicit default.

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.

All existing users have been updated accordingly.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
---
 drivers/s390/char/tape_3590.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/s390/char/tape_3590.c b/drivers/s390/char/tape_3590.c
index a1bafaf73f87..2a2931d303cb 100644
--- a/drivers/s390/char/tape_3590.c
+++ b/drivers/s390/char/tape_3590.c
@@ -1671,7 +1671,7 @@ tape_3590_init(void)
 
 	DBF_EVENT(3, "3590 init\n");
 
-	tape_3590_wq = alloc_workqueue("tape_3590", 0, 0);
+	tape_3590_wq = alloc_workqueue("tape_3590", WQ_PERCPU, 0);
 	if (!tape_3590_wq)
 		return -ENOMEM;
 
-- 
2.51.0

[PATCH v2 2/3] s390/diag324: replace use of system_wq with system_percpu_wq

[Marco Crivellari]

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 renaming system_wq to system_percpu_wq.

queue_work() / queue_delayed_work() mod_delayed_work() will now use the
new per-cpu wq.

The old wq will be kept for a few release cylces.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 arch/s390/kernel/diag/diag324.c | 4 ++--
 1 file changed, 2 insertions(+), 2 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;
-- 
2.51.0

[PATCH v2 3/3] s390: replace use of system_wq with system_dfl_wq

[Marco Crivellari]

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 (replaced by system_percpu_wq), but the
current code does not benefit from it. Because of that, system_wq has been
replaced by system_dfl_wq, the new unbound workqueue.

The old wq will be kept for a few release cylces.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 arch/s390/kernel/hiperdispatch.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/s390/kernel/hiperdispatch.c b/arch/s390/kernel/hiperdispatch.c
index e7b66d046e8d..2507bc3f7757 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_dfl_wq, &hd_capacity_work, HD_DELAY_INTERVAL * hd_delay_factor);
 	hd_update_capacities();
 	return 1;
 }
-- 
2.51.0

Re: [PATCH v2 3/3] s390: replace use of system_wq with system_dfl_wq

[Mete Durlu]

On 9/17/25 5:38 PM, 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 (replaced by system_percpu_wq), but the
> current code does not benefit from it. Because of that, system_wq has been
> replaced by system_dfl_wq, the new unbound workqueue.

That sounds right. Thanks!

> The old wq will be kept for a few release cylces.
> 
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> ---
>   arch/s390/kernel/hiperdispatch.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kernel/hiperdispatch.c b/arch/s390/kernel/hiperdispatch.c
> index e7b66d046e8d..2507bc3f7757 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_dfl_wq, &hd_capacity_work, HD_DELAY_INTERVAL * hd_delay_factor);
>   	hd_update_capacities();
>   	return 1;
>   }


Thank you for making the change from *_percpu_wq to *_dfl_wq.
I am not sure if its too late but you can have my r-b

Reviewed-by: Mete Durlu <meted@linux.ibm.com>

Re: [PATCH v2 3/3] s390: replace use of system_wq with system_dfl_wq

[Marco Crivellari]

On Wed, Sep 24, 2025 at 10:05 AM Mete Durlu <meted@linux.ibm.com> wrote:
> [...]
> Thank you for making the change from *_percpu_wq to *_dfl_wq.
> I am not sure if its too late but you can have my r-b
>
> Reviewed-by: Mete Durlu <meted@linux.ibm.com>

Many thanks, Mete! :-)

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product

Re: [PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Heiko Carstens]

On Wed, Sep 17, 2025 at 05:38:56PM +0200, Marco Crivellari wrote:
> This patchset is the first stone on a refactoring needed in order to
> address the points aforementioned; it will have a positive impact also
> on the cpu isolation, in the long term, moving away percpu workqueue in
> favor to an unbound model.
...
> ---
> Changes in v2:
> - New in the series, 2/3: arch/s390/kernel/hiperdispatch.c does not benefit from
>   system_wq: this has been converted directly to system_unbound_wq.
> - the others system_wq users are converted to system_percpu_wq in patch 3/3
> 
> 
> Marco Crivellari (3):
>   drivers/s390: WQ_PERCPU added to alloc_workqueue users
>   s390/diag324: replace use of system_wq with system_percpu_wq
>   s390: replace use of system_wq with system_dfl_wq
> 
>  arch/s390/kernel/diag/diag324.c  | 4 ++--
>  arch/s390/kernel/hiperdispatch.c | 2 +-
>  drivers/s390/char/tape_3590.c    | 2 +-
>  3 files changed, 4 insertions(+), 4 deletions(-)

Series applied, thanks!

Re: [PATCH v2 0/3] s390: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Marco Crivellari]

On Wed, Sep 24, 2025 at 11:03 AM Heiko Carstens <hca@linux.ibm.com> wrote:
> > Marco Crivellari (3):
> >   drivers/s390: WQ_PERCPU added to alloc_workqueue users
> >   s390/diag324: replace use of system_wq with system_percpu_wq
> >   s390: replace use of system_wq with system_dfl_wq
> >
> >  arch/s390/kernel/diag/diag324.c  | 4 ++--
> >  arch/s390/kernel/hiperdispatch.c | 2 +-
> >  drivers/s390/char/tape_3590.c    | 2 +-
> >  3 files changed, 4 insertions(+), 4 deletions(-)
>
> Series applied, thanks!

Many thanks, Heiko!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product