From: Marco Crivellari Date: Fri, 19 Sep 2025 14:50:39 +0000 (+0200) Subject: rcu: WQ_UNBOUND added to sync_wq workqueue X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=82c427bc935aa5b91d0cabbbc062e71132be2bb8;p=thirdparty%2Fkernel%2Fstable.git rcu: WQ_UNBOUND added to sync_wq workqueue 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 Signed-off-by: Marco Crivellari Reviewed-by: Frederic Weisbecker Signed-off-by: Paul E. McKenney --- diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 01e76c6b2c132..31690ffa452af 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4893,7 +4893,7 @@ 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. */