linux-kernel - Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER

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Date:   Thu, 14 Jan 2021 14:12:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

On Wed, Jan 13, 2021 at 09:28:13PM +0800, Lai Jiangshan wrote:
> On Tue, Jan 12, 2021 at 10:51 PM Peter Zijlstra <peterz@...radead.org> wrote:
> > @@ -4972,9 +4977,11 @@ static void rebind_workers(struct worker
> >          * of all workers first and then clear UNBOUND.  As we're called
> >          * from CPU_ONLINE, the following shouldn't fail.
> >          */
> > -       for_each_pool_worker(worker, pool)
> > +       for_each_pool_worker(worker, pool) {
> >                 WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> >                                                   pool->attrs->cpumask) < 0);
> > +               kthread_set_per_cpu(worker->task, true);
> 
> Will the schedule break affinity in the middle of these two lines due to
> patch4 allowing it and result in Paul's reported splat.

So something like the below _should_ work, except i'm seeing odd WARNs.
I'll prod at it some more.

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2371,6 +2371,7 @@ static int worker_thread(void *__worker)
 	/* tell the scheduler that this is a workqueue worker */
 	set_pf_worker(true);
 woke_up:
+	kthread_parkme();
 	raw_spin_lock_irq(&pool->lock);
 
 	/* am I supposed to die? */
@@ -2428,6 +2429,7 @@ static int worker_thread(void *__worker)
 			move_linked_works(work, &worker->scheduled, NULL);
 			process_scheduled_works(worker);
 		}
+		kthread_parkme();
 	} while (keep_working(pool));
 
 	worker_set_flags(worker, WORKER_PREP);
@@ -4978,9 +4980,9 @@ static void rebind_workers(struct worker
 	 * from CPU_ONLINE, the following shouldn't fail.
 	 */
 	for_each_pool_worker(worker, pool) {
-		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
-						  pool->attrs->cpumask) < 0);
+		kthread_park(worker->task);
 		kthread_set_per_cpu(worker->task, true);
+		kthread_unpark(worker->task);
 	}
 
 	raw_spin_lock_irq(&pool->lock);