linux-kernel - Re: [PATCH v4 15/19] sched: Fix migrate

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Message-ID: <20210305171113.lumrv4m5zdaqhxwn@e107158-lin.cambridge.arm.com>
Date:   Fri, 5 Mar 2021 17:11:13 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
        mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, swood@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vincent.donnefort@....com, tj@...nel.org,
        ouwen210@...mail.com
Subject: Re: [PATCH v4 15/19] sched: Fix migrate_disable() vs rt/dl balancing

On 03/05/21 15:41, Valentin Schneider wrote:
> On 05/03/21 15:56, Peter Zijlstra wrote:
> > On Sat, Dec 26, 2020 at 01:54:45PM +0000, Qais Yousef wrote:
> >>
> >> > +static inline struct task_struct *get_push_task(struct rq *rq)
> >> > +{
> >> > +	struct task_struct *p = rq->curr;
> >>
> >> Shouldn't we verify the class of the task here? The RT task in migration
> >> disabled could have been preempted by a dl or stopper task. Similarly, the dl
> >> task could have been preempted by a stopper task.
> >>
> >> I don't think an RT task should be allowed to push a dl task under any
> >> circumstances?
> >
> > Hmm, quite. Fancy doing a patch?
> 
> Last time we talked about this, I looked into
> 
>   push_rt_task() + find_lowest_rq()
> 
> IIRC, with how
> 
>   find_lowest_rq() + cpupri_find_fitness()
> 
> currently work, find_lowest_rq() should return -1 in push_rt_task() if
> rq->curr is DL (CPUPRI_INVALID). IOW, Migration-Disabled RT tasks shouldn't

[...]

> If you look closely, this is exactly the same as the previous spread
> modulo CPU numbers. IOW, this is (again) a CPU renumbering exercise.

I don't see it a re-numbering exercise. The way I understand it a system
designer doesn't expect their DL task to move because of an RT task. I think we
should try to keep it this way, that's why I asked.

To be fair, I need to look at the code again and understand where I missed that
3rd condition Peter mentioned.

Thanks

--
Qais Yousef