From: Marco Crivellari Date: Fri, 19 Sep 2025 14:50:38 +0000 (+0200) Subject: rcu: WQ_PERCPU added to alloc_workqueue users X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=499d48f75b230522f4aa5aa4b9cc3c5b1594e1af;p=thirdparty%2Fkernel%2Fstable.git rcu: WQ_PERCPU added to alloc_workqueue users 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 patch adds a new WQ_PERCPU flag to explicitly request the use of the per-CPU behavior. Both flags coexist for one release cycle to allow callers to transition their calls. 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. All existing users have been updated accordingly. 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 1291e0761d70a..01e76c6b2c132 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4890,7 +4890,7 @@ void __init rcu_init(void) rcutree_online_cpu(cpu); /* Create workqueue for Tree SRCU and for expedited GPs. */ - rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0); + 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);