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 <t...@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivell...@suse.com>
Reviewed-by: Frederic Weisbecker <frede...@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
