linux-kernel - Re: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

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Message-ID: <ba994b0f4b4402612d2e14552882c29dca83ad5f.camel@gmx.de>
Date:   Sun, 02 Apr 2023 04:40:20 +0200
From:   Mike Galbraith <efault@....de>
To:     Hillf Danton <hdanton@...a.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        corbet@....net, qyousef@...alina.io, joshdon@...gle.com,
        timj@....org, kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org, linux-mm@...ck.org,
        joel@...lfernandes.org
Subject: Re: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

On Sun, 2023-04-02 at 07:23 +0800, Hillf Danton wrote:
> On 31 Mar 2023 17:26:51 +0200 Vincent Guittot <vincent.guittot@...aro.org>
> >
> > I wanted to stress this situation with a simple use case but it seems
> > that even without changing the slice, there is a fairness problem:
> >
> > Task A always run
> > Task B loops on : running 1ms then sleeping 1ms
> > default nice and latency nice prio bot both
> > each task should get around 50% of the time.
> >
> > The fairness is ok with tip/sched/core
> > but with eevdf, Task B only gets around 30%
>
> Convincing evidence for glitch in wakeup preempt.

If you turn on PLACE_BONUS, it'll mimic FAIR_SLEEPERS.. but if you then
do some testing, you'll probably turn it right back off.

The 50/50 split in current code isn't really any more fair, as soon as
you leave the tiny bubble of fairness, it's not the least bit fair.
Nor is that tiny bubble all rainbows and unicorns, it brought with it
benchmark wins and losses, like everything that changes more than
comments, its price being service latency variance.

The short term split doesn't really mean all that much, some things
will like the current fair-bubble better, some eevdf virtual deadline
math and its less spiky service.  We'll see.

I'm kinda hoping eevdf works out, FAIR_SLEEPERS is quite annoying to
squabble with.

	-Mike