linux-kernel - Re: [RFC PATCH 3/6] sched/dl: Try better placement even for deadline tasks that do not block

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Date:   Tue, 9 Jul 2019 15:24:36 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Quentin Perret <quentin.perret@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [RFC PATCH 3/6] sched/dl: Try better placement even for
 deadline tasks that do not block

Hi Peter,

On Mon, 8 Jul 2019 15:55:36 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, May 06, 2019 at 06:48:33AM +0200, Luca Abeni wrote:
> > @@ -1223,8 +1250,17 @@ static void update_curr_dl(struct rq *rq)
> >  			dl_se->dl_overrun = 1;
> >  
> >  		__dequeue_task_dl(rq, curr, 0);
> > -		if (unlikely(dl_se->dl_boosted
> > || !start_dl_timer(curr)))
> > +		if (unlikely(dl_se->dl_boosted
> > || !start_dl_timer(curr))) { enqueue_task_dl(rq, curr,
> > ENQUEUE_REPLENISH); +#ifdef CONFIG_SMP
> > +		} else if (dl_se->dl_adjust) {
> > +			if (rq->migrating_task == NULL) {
> > +				queue_balance_callback(rq,
> > &per_cpu(dl_migrate_head, rq->cpu), migrate_dl_task);  
> 
> I'm not entirely sure about this one.
> 
> That is, we only do those callbacks from:
> 
>   schedule_tail()
>   __schedule()
>   rt_mutex_setprio()
>   __sched_setscheduler()
> 
> and the above looks like it can happen outside of those.

Sorry, I did not know the constraints or requirements for using
queue_balance_callback()...

I used it because I wanted to trigger a migration from
update_curr_dl(), but invoking double_lock_balance() from this function
obviously resulted in a warning. So, I probably misunderstood the
purpose of the balance callback API, and I misused it.

What would have been the "right way" to trigger a migration for a task
when it is throttled?


> 
> The pattern in those sites is:
> 
> 	rq_lock();
> 	... do crap that leads to queue_balance_callback()
> 	rq_unlock()
> 	if (rq->balance_callback) {
> 		raw_spin_lock_irqsave(rq->lock, flags);
> 		... do callbacks
> 		raw_spin_unlock_irqrestore(rq->lock, flags);
> 	}
> 
> So I suppose can catch abuse of this API by doing something like the
> below; can you validate?

Sorry; right now I cannot run tests on big.LITTLE machines... 
Maybe Dietmar (added in cc), who is working on mainlining this patcset,
can test?



				Thanks,
					Luca

> 
> ---
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index aaca0e743776..89e615f1eae6 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1134,6 +1134,14 @@ static inline void rq_pin_lock(struct rq *rq,
> struct rq_flags *rf) rf->cookie = lockdep_pin_lock(&rq->lock);
>  
>  #ifdef CONFIG_SCHED_DEBUG
> +#ifdef CONFIG_SMP
> +	/*
> +	 * There should not be pending callbacks at the start of
> rq_lock();
> +	 * all sites that handle them flush them at the end.
> +	 */
> +	WARN_ON_ONCE(rq->balance_callback);
> +#endif
> +
>  	rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
>  	rf->clock_update_flags = 0;
>  #endif