linux-kernel - Re: [RFC][PATCH 04/13] sched/{rt,deadline}: Fix set_next_task vs pick_next

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Message-ID: <20190729144025.GD31381@hirez.programming.kicks-ass.net>
Date:   Mon, 29 Jul 2019 16:40:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, luca.abeni@...tannapisa.it,
        bristot@...hat.com, balsini@...roid.com, dvyukov@...gle.com,
        tglx@...utronix.de, vpillai@...italocean.com, rostedt@...dmis.org
Subject: Re: [RFC][PATCH 04/13] sched/{rt,deadline}: Fix set_next_task vs
 pick_next_task

On Mon, Jul 29, 2019 at 03:17:01PM +0200, Juri Lelli wrote:
> On 29/07/19 15:04, Peter Zijlstra wrote:

> > Now, looking at it, this also doesn't do push balancing when we
> > re-select the same task, even though we really should be doing it. So I
> > suppose not adding the condition, and always doing the push balance,
> > while wasteful, is not wrong.
> 
> Right, also because deadline_queue_push_tasks() already checks if there
> are tasks to potentially push around before queuing the balance
> callback.

Yes, but in the overloaded case, where there is always a task to push,
but nowhere to push it to, we can waste a 'lot' of time looking for
naught in case of extra pushes.

So in that regard the check you reference is not sufficient.

Anyway, let me change this for now.