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

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Date:   Sat, 16 Jan 2021 19:46:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, 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 Sun, Jan 17, 2021 at 12:14:34AM +0800, Lai Jiangshan wrote:

> BP:                 AP:                  worker:
> cpus_write_lock()
> bringup_cpu()                            work_item_func()
>   bringup_wait_for_ap                      get_online_cpus()
>                     kthread_park(worker)

Thanks, pictures are easier. Agreed, that a problem.

I've also found another problem I think.  rescuer_thread becomes part of
for_each_pool_worker() between worker_attach_to_pool() and
worker_detach_from_pool(), so it would try and do kthread_park() on
rescuer, when things align. And rescuer_thread() doesn't have a
kthread_parkme().

And we already rely on this 'ugly' thing of first doing
kthread_set_per_cpu() and fixing up the affinity later for the rescuer.

Let me restart the SRCU-P testing with the below delta applied.

---
 kernel/workqueue.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1db769b116a1..894bb885b40b 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2368,7 +2368,6 @@ 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? */
@@ -2426,7 +2425,7 @@ static int worker_thread(void *__worker)
 			move_linked_works(work, &worker->scheduled, NULL);
 			process_scheduled_works(worker);
 		}
-	} while (keep_working(pool) && !kthread_should_park());
+	} while (keep_working(pool));
 
 	worker_set_flags(worker, WORKER_PREP);
 sleep:
@@ -2438,12 +2437,9 @@ static int worker_thread(void *__worker)
 	 * event.
 	 */
 	worker_enter_idle(worker);
-	set_current_state(TASK_IDLE);
+	__set_current_state(TASK_IDLE);
 	raw_spin_unlock_irq(&pool->lock);
-
-	if (!kthread_should_park())
-		schedule();
-
+	schedule();
 	goto woke_up;
 }
 
@@ -4979,9 +4975,9 @@ static void rebind_workers(struct worker_pool *pool)
 	 * from CPU_ONLINE, the following shouldn't fail.
 	 */
 	for_each_pool_worker(worker, pool) {
-		WARN_ON_ONCE(kthread_park(worker->task) < 0);
 		kthread_set_per_cpu(worker->task, pool->cpu);
-		kthread_unpark(worker->task);
+		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
+						  pool->attrs->cpumask) < 0);
 	}
 
 	raw_spin_lock_irq(&pool->lock);