[PATCH 0/1] sched: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[PATCH 0/1] sched: replace wq users and add WQ_PERCPU to alloc_workqueue() users

[Marco Crivellari]

Hi!

Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:

        "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
        https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/

=== Current situation: problems ===

Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.

This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
        schedule_delayed_work(, 0);

This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:

        schedule_delayed_work(, 1);

Will move the timer on an housekeeping CPU, and schedule the work there.

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.

=== Plan and future plans ===

This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.

These are the main steps:
1)  API refactoring (that this patch is introducing)
    -   Make more clear and uniform the system wq names, both per-cpu and
        unbound. This to avoid any possible confusion on what should be
        used.

    -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
        introduced in this patchset and used on all the callers that are not
        currently using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a future release cycle.

        Most users don't need to be per-cpu, because they don't have
        locality requirements, because of that, a next future step will be
        make "unbound" the default behavior.

2)  Check who really needs to be per-cpu
    -   Remove the WQ_PERCPU flag when is not strictly required.

3)  Add a new API (prefer local cpu)
    -   There are users that don't require a local execution, like mentioned
        above; despite that, local execution yeld to performance gain.

        This new API will prefer the local execution, without requiring it.

=== Introduced Changes by this series ===

1) [P 1] Replace use of system_unbound_wq

        system_unbound_wq is to be used when locality is not required.

        Because of that, system_wq has been renamed in system_percpu_wq, and
        system_unbound_wq has been renamed in system_dfl_wq.


=== For Maintainers ===

There are prerequisites for this series, already merged in the master branch.
The commits are:

128ea9f6ccfb6960293ae4212f4f97165e42222d ("workqueue: Add system_percpu_wq and
system_dfl_wq")

930c2ea566aff59e962c50b2421d5fcc3b98b8be ("workqueue: Add new WQ_PERCPU flag")


Thanks!

Marco Crivellari (1):
  sched: replace use of system_unbound_wq with system_dfl_wq

 kernel/sched/core.c | 4 ++--
 kernel/sched/ext.c  | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

-- 
2.51.0

[PATCH 1/1] sched: replace use of system_unbound_wq with system_dfl_wq

[Marco Crivellari]

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.

system_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

Adding system_dfl_wq to encourage its use when unbound work should be used.

queue_work() / queue_delayed_work() / mod_delayed_work() will now use the
new unbound wq: whether the user still use the old wq a warn will be
printed along with a wq redirect to the new one.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 kernel/sched/core.c | 4 ++--
 kernel/sched/ext.c  | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c81cf642dba0..baa096e543f1 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5772,7 +5772,7 @@ static void sched_tick_remote(struct work_struct *work)
 	os = atomic_fetch_add_unless(&twork->state, -1, TICK_SCHED_REMOTE_RUNNING);
 	WARN_ON_ONCE(os == TICK_SCHED_REMOTE_OFFLINE);
 	if (os == TICK_SCHED_REMOTE_RUNNING)
-		queue_delayed_work(system_unbound_wq, dwork, HZ);
+		queue_delayed_work(system_dfl_wq, dwork, HZ);
 }
 
 static void sched_tick_start(int cpu)
@@ -5791,7 +5791,7 @@ static void sched_tick_start(int cpu)
 	if (os == TICK_SCHED_REMOTE_OFFLINE) {
 		twork->cpu = cpu;
 		INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
-		queue_delayed_work(system_unbound_wq, &twork->work, HZ);
+		queue_delayed_work(system_dfl_wq, &twork->work, HZ);
 	}
 }
 
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 66bcd40a28ca..b1b79143e391 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -3514,7 +3514,7 @@ static void scx_watchdog_workfn(struct work_struct *work)
 
 		cond_resched();
 	}
-	queue_delayed_work(system_unbound_wq, to_delayed_work(work),
+	queue_delayed_work(system_dfl_wq, to_delayed_work(work),
 			   scx_watchdog_timeout / 2);
 }
 
@@ -5403,7 +5403,7 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
 
 	WRITE_ONCE(scx_watchdog_timeout, timeout);
 	WRITE_ONCE(scx_watchdog_timestamp, jiffies);
-	queue_delayed_work(system_unbound_wq, &scx_watchdog_work,
+	queue_delayed_work(system_dfl_wq, &scx_watchdog_work,
 			   scx_watchdog_timeout / 2);
 
 	/*
-- 
2.51.0

Re: [PATCH 1/1] sched: replace use of system_unbound_wq with system_dfl_wq

[Tejun Heo]

On Fri, Sep 05, 2025 at 11:09:28AM +0200, Marco Crivellari wrote:
> 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.
> 
> system_unbound_wq should be the default workqueue so as not to enforce
> locality constraints for random work whenever it's not required.
> 
> Adding system_dfl_wq to encourage its use when unbound work should be used.
> 
> queue_work() / queue_delayed_work() / mod_delayed_work() will now use the
> new unbound wq: whether the user still use the old wq a warn will be
> printed along with a wq redirect to the new one.
> 
> The old system_unbound_wq will be kept for a few release cycles.
> 
> Suggested-by: Tejun Heo <tj@kernel.org>
> Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun