From: Marco Crivellari <marco.crivellari@suse.com>
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 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 <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1291e0761d70ab..c51c4a0af6aa5e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4890,10 +4890,10 @@ 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);
+ sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_PERCPU, 0);
WARN_ON(!sync_wq);
/* Respect if explicitly disabled via a boot parameter. */
--
2.40.1
Le Thu, Sep 18, 2025 at 03:17:52AM -0700, Paul E. McKenney a écrit : > From: Marco Crivellari <marco.crivellari@suse.com> > > 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 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 <tj@kernel.org> > Signed-off-by: Marco Crivellari <marco.crivellari@suse.com> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > --- > kernel/rcu/tree.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 1291e0761d70ab..c51c4a0af6aa5e 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -4890,10 +4890,10 @@ 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); > + sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_PERCPU, 0); > WARN_ON(!sync_wq); sync_wq could be unbound as it's not dealing with per-cpu data. Thanks. > > /* Respect if explicitly disabled via a boot parameter. */ > -- > 2.40.1 > > -- Frederic Weisbecker SUSE Labs
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