linux-kernel - Re: [PATCH v2 6/6] sched/deadline: Implement fallback mechanism for !fit case

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Message-ID: <20200427161715.3dd3a148@nowhere>
Date:   Mon, 27 Apr 2020 16:17:15 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] sched/deadline: Implement fallback mechanism for
 !fit case

Hi Juri,

On Mon, 27 Apr 2020 15:34:38 +0200
Juri Lelli <juri.lelli@...hat.com> wrote:

> Hi,
> 
> On 27/04/20 10:37, Dietmar Eggemann wrote:
> > From: Luca Abeni <luca.abeni@...tannapisa.it>
> > 
> > When a task has a runtime that cannot be served within the
> > scheduling deadline by any of the idle CPU (later_mask) the task is
> > doomed to miss its deadline.
> > 
> > This can happen since the SCHED_DEADLINE admission control
> > guarantees only bounded tardiness and not the hard respect of all
> > deadlines. In this case try to select the idle CPU with the largest
> > CPU capacity to minimize tardiness.
> > 
> > Signed-off-by: Luca Abeni <luca.abeni@...tannapisa.it>
> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
[...]
> > -		if (!cpumask_empty(later_mask))
> > -			return 1;
> > +		if (cpumask_empty(later_mask))
> > +			cpumask_set_cpu(max_cpu, later_mask);  
> 
> Think we touched upon this during v1 review, but I'm (still?)
> wondering if we can do a little better, still considering only free
> cpus.
> 
> Can't we get into a situation that some of the (once free) big cpus
> have been occupied by small tasks and now a big task enters the
> system and it only finds small cpus available, were it could have fit
> into bigs if small tasks were put onto small cpus?
> 
> I.e., shouldn't we always try to best fit among free cpus?

Yes; there was an additional patch that tried schedule each task on the
slowest core where it can fit, to address this issue.
But I think it will go in a second round of patches.



			Luca