linux-kernel - Re: [RFC PATCH 1/6] sched/fair: check_preempt_wakeup: Fix assumption on the default policy

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Message-ID: <alpine.DEB.2.20.1609210042330.5476@nanos>
Date:   Wed, 21 Sep 2016 00:56:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     Julien Desfossez <jdesfossez@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>,
        rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>,
        daolivei <daolivei@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/6] sched/fair: check_preempt_wakeup: Fix assumption
 on the default policy

On Tue, 20 Sep 2016, Mathieu Desnoyers wrote:
> So what is then puzzling us is this:
> 
> rt_mutex_setprio()
> 
>         if (dl_prio(prio)) {
>                 struct task_struct *pi_task = rt_mutex_get_top_task(p);
>                 if (!dl_prio(p->normal_prio) ||
>                     (pi_task && dl_entity_preempt(&pi_task->dl, &p->dl))) {
>                         p->dl.dl_boosted = 1;
>                         queue_flag |= ENQUEUE_REPLENISH;
>                 } else
>                         p->dl.dl_boosted = 0;
>                 p->sched_class = &dl_sched_class;
>         } else if (rt_prio(prio)) {
>                 if (dl_prio(oldprio))
>                         p->dl.dl_boosted = 0;
>                 if (oldprio < prio)
>                         queue_flag |= ENQUEUE_HEAD;
>                 p->sched_class = &rt_sched_class;
>         } else {
>                 if (dl_prio(oldprio))
>                         p->dl.dl_boosted = 0;
>                 if (rt_prio(oldprio))
>                         p->rt.timeout = 0;
>                 p->sched_class = &fair_sched_class;
>         }
> 
> So in the 3rd block, this is the case where we inherit a
> new prio which is neither LD nor RT, so it's "fair".
> 
> If we try to assign a fair prio to a task of DL or RT
> prio, the dl_boosted is set to 0, or the rt timeout is
> set to 0. However, we do change the sched_class of the
> target task to &fair_sched_class.
> 
> This code path seems to imply that a RT or DL task can
> change sched_class to "fair". Indeed, it makes no sense,
> so I have the feeling we're missing something important
> here.

Yes. Look at the call site of this. This is called in two cases:

 - The task blocked on a lock pushes the task owning the lock and in that
   case it is guaranteed that the blocked task is in the same or in a
   higher scheduling class. We have no support for inverse PI (yet).

 - The task which got boosted deboosts itself after releasing the lock. In
   that case it falls back to its original class/priority/bandwidth.

Hope that helps.

> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Cc: Steven Rostedt (Red Hat) <rostedt@...dmis.org>
> >> Cc: Thomas Gleixner <tglx@...utronix.de>
> >> Cc: Ingo Molnar <mingo@...hat.com>
> >> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> >> Signed-off-by: Julien Desfossez <jdesfossez@...icios.com>
> > 
> > Who wrote the patch?
> 
> Julien is the author.

So the SOB chain is wrong because you neither authored the patch nor you
conveyed it.

Thanks,

	tglx