[PATCH] workqueue: don't use wq_select_unbound_cpu() for bound works

[PATCH] workqueue: don't use wq_select_unbound_cpu() for bound works

[Daniel Jordan]

From: Hillf Danton <hdanton@sina.com>

wq_select_unbound_cpu() is designed for unbound workqueues only, but
it's wrongly called when using a bound workqueue too.

Fixing this ensures work queued to a bound workqueue with
cpu=WORK_CPU_UNBOUND always runs on the local CPU.

Before, that would happen only if wq_unbound_cpumask happened to include
it (likely almost always the case), or was empty, or we got lucky with
forced round-robin placement.  So restricting
/sys/devices/virtual/workqueue/cpumask to a small subset of a machine's
CPUs would cause some bound work items to run unexpectedly there.

Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs")
Signed-off-by: Hillf Danton <hdanton@sina.com>
[dj: massage changelog]
Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: linux-kernel@vger.kernel.org
---
 kernel/workqueue.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index cfc923558e04..6978892b0d8a 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1411,14 +1411,16 @@ static void __queue_work(int cpu, struct workqueue_struct *wq,
 		return;
 	rcu_read_lock();
 retry:
-	if (req_cpu == WORK_CPU_UNBOUND)
-		cpu = wq_select_unbound_cpu(raw_smp_processor_id());
-
 	/* pwq which will be used unless @work is executing elsewhere */
-	if (!(wq->flags & WQ_UNBOUND))
-		pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
-	else
+	if (wq->flags & WQ_UNBOUND) {
+		if (req_cpu == WORK_CPU_UNBOUND)
+			cpu = wq_select_unbound_cpu(raw_smp_processor_id());
 		pwq = unbound_pwq_by_node(wq, cpu_to_node(cpu));
+	} else {
+		if (req_cpu == WORK_CPU_UNBOUND)
+			cpu = raw_smp_processor_id();
+		pwq = per_cpu_ptr(wq->cpu_pwqs, cpu);
+	}
 
 	/*
 	 * If @work was previously on a different pool, it might still be
-- 
2.24.1

Re: [PATCH] workqueue: don't use wq_select_unbound_cpu() for bound works

[Tejun Heo]

On Fri, Jan 24, 2020 at 08:14:45PM -0500, Daniel Jordan wrote:
> From: Hillf Danton <hdanton@sina.com>
> 
> wq_select_unbound_cpu() is designed for unbound workqueues only, but
> it's wrongly called when using a bound workqueue too.
> 
> Fixing this ensures work queued to a bound workqueue with
> cpu=WORK_CPU_UNBOUND always runs on the local CPU.
> 
> Before, that would happen only if wq_unbound_cpumask happened to include
> it (likely almost always the case), or was empty, or we got lucky with
> forced round-robin placement.  So restricting
> /sys/devices/virtual/workqueue/cpumask to a small subset of a machine's
> CPUs would cause some bound work items to run unexpectedly there.
> 
> Fixes: ef557180447f ("workqueue: schedule WORK_CPU_UNBOUND work on wq_unbound_cpumask CPUs")
> Signed-off-by: Hillf Danton <hdanton@sina.com>
> [dj: massage changelog]
> Signed-off-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: linux-kernel@vger.kernel.org

Applied to wq/for-5.6-fixes.

Thanks.

-- 
tejun