rcu: replace system_wq and add WQ_PERCPU or WQ_UNBOUND to alloc_workqueue users

[PATCH v2 3/3] rcu: WQ_UNBOUND added to sync_wq workqueue

Posted by Marco Crivellari 4 months, 3 weeks ago

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 <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 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

Re: [PATCH v2 3/3] rcu: WQ_UNBOUND added to sync_wq workqueue

Posted by Frederic Weisbecker 4 months, 2 weeks ago

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 <tj@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>


> ---
>  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

Re: [PATCH v2 3/3] rcu: WQ_UNBOUND added to sync_wq workqueue

Posted by Paul E. McKenney 4 months, 2 weeks ago

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 <tj@kernel.org>
> > Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
> 
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

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

Re: [PATCH v2 3/3] rcu: WQ_UNBOUND added to sync_wq workqueue

Posted by Marco Crivellari 4 months, 2 weeks ago

On Mon, Sep 22, 2025 at 4:27 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> Applied, thank you both!
>
> I will push these out on my next rebase.

Many thanks, Paul!

-- 

Marco Crivellari

L3 Support Engineer, Technology & Product