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Date:   Sun, 17 Jan 2021 10:54:07 +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 Sat, Jan 16, 2021 at 07:46:47PM +0100, Peter Zijlstra wrote:
> 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);

In the roughly 80 instances of 18*SRCU-P since sending this, I've got
one sched_cpu_dying splat about a stray kworker, so somthing isn't
right.

My intention was to not think today, so I'll delay that until tomorrow.

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