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:   Mon, 8 Jul 2019 15:55:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Luca Abeni <luca.abeni@...tannapisa.it>
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>
Subject: Re: [RFC PATCH 3/6] sched/dl: Try better placement even for deadline
 tasks that do not block

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.

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?

---

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