On Mon, Sep 22, 2025 at 04:14:46PM +0200, Frederic Weisbecker wrote:
> Le Fri, Sep 19, 2025 at 04:50:39PM +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.
> >
> > 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 change add the WQ_UNBOUND flag to sync_wq, to make explicit this
> > workqueue can be unbound and that it does not benefit from per-cpu work.
> >
> > 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.
> >
> > Suggested-by: Tejun Heo <[email protected]>
> > Signed-off-by: Marco Crivellari <[email protected]>
>
> Reviewed-by: Frederic Weisbecker <[email protected]>
Applied, thank you both!
I will push these out on my next rebase.
Thanx, Paul
> > ---
> > kernel/rcu/tree.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 4f3175df5999..7137723f8f95 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -4888,7 +4888,7 @@ void __init rcu_init(void)
> > rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM | WQ_PERCPU, 0);
> > WARN_ON(!rcu_gp_wq);
> >
> > - sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM, 0);
> > + sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
> > WARN_ON(!sync_wq);
> >
> > /* Respect if explicitly disabled via a boot parameter. */
> > --
> > 2.51.0
> >
>
> --
> Frederic Weisbecker
> SUSE Labs
