linux-kernel - Re: [PATCH v4 1/3] Workqueue: add system_percpu_wq and system_dfl

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Message-ID: <CAAofZF7YSXCTvUJmXNBZM3eSj3z+j7TQPQBwmwYF8v_nKDFHsQ@mail.gmail.com>
Date: Fri, 13 Jun 2025 15:19:56 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>, 
	Lai Jiangshan <jiangshanlai@...il.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v4 1/3] Workqueue: add system_percpu_wq and system_dfl_wq

Hi Frederic,

I let the wq allocation together with the wq logic changes.
But if it's better to allocate directly here when we add the wq(s), I
will do so.

Thank you.



On Fri, Jun 13, 2025 at 3:05 PM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> Le Thu, Jun 12, 2025 at 03:33:33PM +0200, Marco Crivellari a écrit :
> > 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.
> >
> > system_unbound_wq should be the default workqueue so as not to enforce
> > locality constraints for random work whenever it's not required.
> >
> > Adding system_dfl_wq to encourage its use when unbound work should be used.
> >
> > Suggested-by: Tejun Heo <tj@...nel.org>
> > Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
> > ---
> >  include/linux/workqueue.h | 8 +++++---
> >  kernel/workqueue.c        | 4 ++++
> >  2 files changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> > index 6e30f275da77..502ec4a5e32c 100644
> > --- a/include/linux/workqueue.h
> > +++ b/include/linux/workqueue.h
> > @@ -427,7 +427,7 @@ enum wq_consts {
> >  /*
> >   * System-wide workqueues which are always present.
> >   *
> > - * system_wq is the one used by schedule[_delayed]_work[_on]().
> > + * system_percpu_wq is the one used by schedule[_delayed]_work[_on]().
> >   * Multi-CPU multi-threaded.  There are users which expect relatively
> >   * short queue flush time.  Don't queue works which can run for too
> >   * long.
> > @@ -438,7 +438,7 @@ enum wq_consts {
> >   * system_long_wq is similar to system_wq but may host long running
> >   * works.  Queue flushing might take relatively long.
> >   *
> > - * system_unbound_wq is unbound workqueue.  Workers are not bound to
> > + * system_dfl_wq is unbound workqueue.  Workers are not bound to
> >   * any specific CPU, not concurrency managed, and all queued works are
> >   * executed immediately as long as max_active limit is not reached and
> >   * resources are available.
> > @@ -455,10 +455,12 @@ enum wq_consts {
> >   * system_bh[_highpri]_wq are convenience interface to softirq. BH work items
> >   * are executed in the queueing CPU's BH context in the queueing order.
> >   */
> > -extern struct workqueue_struct *system_wq;
> > +extern struct workqueue_struct *system_wq; /* use system_percpu_wq, this will be removed */
> > +extern struct workqueue_struct *system_percpu_wq;
> >  extern struct workqueue_struct *system_highpri_wq;
> >  extern struct workqueue_struct *system_long_wq;
> >  extern struct workqueue_struct *system_unbound_wq;
> > +extern struct workqueue_struct *system_dfl_wq;
> >  extern struct workqueue_struct *system_freezable_wq;
> >  extern struct workqueue_struct *system_power_efficient_wq;
> >  extern struct workqueue_struct *system_freezable_power_efficient_wq;
> > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> > index 97f37b5bae66..7a3f53a9841e 100644
> > --- a/kernel/workqueue.c
> > +++ b/kernel/workqueue.c
> > @@ -505,12 +505,16 @@ static struct kthread_worker *pwq_release_worker __ro_after_init;
> >
> >  struct workqueue_struct *system_wq __ro_after_init;
> >  EXPORT_SYMBOL(system_wq);
> > +struct workqueue_struct *system_percpu_wq __ro_after_init;
> > +EXPORT_SYMBOL(system_percpu_wq);
> >  struct workqueue_struct *system_highpri_wq __ro_after_init;
> >  EXPORT_SYMBOL_GPL(system_highpri_wq);
> >  struct workqueue_struct *system_long_wq __ro_after_init;
> >  EXPORT_SYMBOL_GPL(system_long_wq);
> >  struct workqueue_struct *system_unbound_wq __ro_after_init;
> >  EXPORT_SYMBOL_GPL(system_unbound_wq);
> > +struct workqueue_struct *system_dfl_wq __ro_after_init;
> > +EXPORT_SYMBOL_GPL(system_dfl_wq);
> >  struct workqueue_struct *system_freezable_wq __ro_after_init;
> >  EXPORT_SYMBOL_GPL(system_freezable_wq);
> >  struct workqueue_struct *system_power_efficient_wq __ro_after_init;
>
> Shouldn't you allocate system_percpu_wq and system_dfl_wq in
> workqueue_init_early() ?
>
> And yes I think we should allocate them and not make them a pointer to
> system_wq and system_unbound_wq, this way you can more easily
> warn deprecated uses of system_wq and system_unbound_wq in the future
> after upcoming merge windows.
>
> Thanks.
>
> > --
> > 2.49.0
> >
>
> --
> Frederic Weisbecker
> SUSE Labs



--

Marco Crivellari

L3 Support Engineer, Technology & Product




marco.crivellari@...e.com