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

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Message-ID: <20190508101414.1c968810@nowhere>
Date:   Wed, 8 May 2019 10:14:14 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        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

Hi Juri,

On Wed, 8 May 2019 10:01:16 +0200
Juri Lelli <juri.lelli@...hat.com> wrote:

> Hi Luca,
> 
> On 06/05/19 06:48, Luca Abeni wrote:
> > From: luca abeni <luca.abeni@...tannapisa.it>
> > 
> > Currently, the scheduler tries to find a proper placement for
> > SCHED_DEADLINE tasks when they are pushed out of a core or when
> > they wake up. Hence, if there is a single SCHED_DEADLINE task
> > that never blocks and wakes up, such a task is never migrated to
> > an appropriate CPU core, but continues to execute on its original
> > core.
> > 
> > This commit addresses the issue by trying to migrate a
> > SCHED_DEADLINE task (searching for an appropriate CPU core) the
> > first time it is throttled.  
> 
> Why we failed to put the task on a CPU with enough (max) capacity
> right after it passed admission control? The very first time the task
> was scheduled I mean.

I think the currently executing task cannot be pushed out of a
CPU/core, right?

So, if a task switches from SCHED_OTHER to SCHED_DEADLINE while it is
executing on a fast core, the only way to migrate it would be to
preempt it (by using the stop_sched_class, I think), no?
(the typical situation here is a "cpu hog" task that switches from
SCHED_OTHER to SCHED_DEADLINE, and it is the only SCHED_DEADLINE
task... The task never blocks, so push/pull functions are never invoked)

Or am I missing something?



			Thanks,
				Luca