linux-kernel - Re: [PATCH v3] sched/deadline: Fix the intention to re-evalute tick dependency for offline cpu

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Message-ID: <20160819132549.GG10121@twins.programming.kicks-ass.net>
Date:   Fri, 19 Aug 2016 15:25:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Luca Abeni <luca.abeni@...tn.it>,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v3] sched/deadline: Fix the intention to re-evalute tick
 dependency for offline cpu

On Fri, Aug 12, 2016 at 05:24:03PM +0800, Wanpeng Li wrote:

> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index d091f4a..ce0fb00 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -641,6 +641,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>  		goto unlock;
>  	}
>  
> +#ifdef CONFIG_SMP
> +	if (unlikely(!rq->online))
> +		goto offline;
> +#endif
> +
>  	enqueue_task_dl(rq, p, ENQUEUE_REPLENISH);
>  	if (dl_task(rq->curr))
>  		check_preempt_curr_dl(rq, p, 0);
> @@ -648,6 +653,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>  		resched_curr(rq);
>  
>  #ifdef CONFIG_SMP
> +offline:
>  	/*
>  	 * Perform balancing operations here; after the replenishments.  We
>  	 * cannot drop rq->lock before this, otherwise the assertion in
> @@ -659,6 +665,7 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
>  	 * XXX figure out if select_task_rq_dl() deals with offline cpus.
>  	 */
>  	if (unlikely(!rq->online)) {
> +		replenish_dl_entity(dl_se, dl_se);
>  		lockdep_unpin_lock(&rq->lock, rf.cookie);
>  		rq = dl_task_offline_migration(rq, p);

So I don't like this, even if it magically works. With this we end up
calling dl_task_offline_migration() -> deactivate_task() while the task
isn't on the runqueue at all.

>  		rf.cookie = lockdep_pin_lock(&rq->lock);
> -- 
> 1.9.1
>