linux-kernel - Re: [PATCH v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP

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Message-ID: <20230523225831.60d75b38@nowhere>
Date:   Tue, 23 May 2023 22:58:31 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Vineeth Remanan Pillai <vineeth@...byteword.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v3 2/5] sched/deadline: Fix reclaim inaccuracy with SMP

Hi,

sorry for the late reply.

On Mon, 22 May 2023 15:22:52 -0400
Vineeth Remanan Pillai <vineeth@...byteword.org> wrote:
[...]
> > But when I use
> >         dq = -(max{u_i, (Umax - Uinact - Uextra)} / Umax) * dt
> > everything works as expected, the 4 tasks reclaim 95% of the CPU
> > time and my shell is still active...
> > (so, I cannot reproduce the starvation issue with this equation)
> >  
> Sorry about this confusion, yes you are right, there is no stall with
> this equation. The only issue is the lesser reclaim when the load is
> less and tasks have different bandwidth requirements.
> 
> > So, I now think the second one is the correct equation to be used.
> >  
> Thanks for confirming.
> 
> I think it probably makes sense to get the fix for the equation to go
> in as a first step and then we can investigate more about the second
> issue (less reclaiming with less load and different bandwidth) and
> fix it separately. What do you think?

I fully agree. If you split this change in a first patch, IMHO it
can be applied.

BTW, I tried changing the equation without introducing div64, and it
seems to me that it works well... So, if removing the bw_ratio
approximation is needed, I think you can do it in a second patch (so,
the first patch changes the reclaiming equation, and the second one
introduces div64)


			Thanks,
				Luca

> I shall send the next iteration
> with the fix for the equation alone if its okay with you.
> 
> Thanks,
> Vineeth