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

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Message-ID: <20230414111808.GA144166@lorien.usersys.redhat.com>
Date:   Fri, 14 Apr 2023 07:18:08 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, corbet@....net,
        qyousef@...alina.io, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, pjt@...gle.com, pavel@....cz,
        qperret@...gle.com, tim.c.chen@...ux.intel.com, joshdon@...gle.com,
        timj@....org, kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org, efault@....de
Subject: Re: [PATCH 14/17] sched/eevdf: Better handle mixed slice length

On Wed, Apr 05, 2023 at 04:05:55PM -0400 Joel Fernandes wrote:
> On Wed, Apr 5, 2023 at 4:36 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Apr 04, 2023 at 01:50:50PM +0000, Joel Fernandes wrote:
> > > On Tue, Apr 04, 2023 at 11:29:36AM +0200, Peter Zijlstra wrote:
> >
> > > > Heh, this is actually the correct behaviour. If you have a u=1 and a
> > > > u=.5 task, you should distribute time on a 2:1 basis, eg. 67% vs 33%.
> > >
> > > Splitting like that sounds like starvation of the sleeper to me. If something
> > > sleeps a lot, it will get even less CPU time on an average than it would if
> > > there was no contention from the u=1 task.
> >
> > No, sleeping, per definition, means you're not contending for CPU. What
> > CFS does, giving them a little boost, is strictly yuck and messes with
> > latency -- because suddenly you have a task that said it wasn't
> > competing appear as if it were, but you didn't run it (how could you, it
> > wasn't there to run) -- but it still needs to catch up.
> >
> > The reason it does that, is mostly because at the time we didn't want to
> > do the whole lag thing -- it's somewhat heavy on the u64 mults and 32bit
> > computing was still a thing :/ So hacks happened.
> 
> Also you have the whole "boost tasks" that sleep a lot with CFS right?
>  Like a task handling user input sleeps a lot, but when it wakes up,
> it gets higher dynamic priority as its vruntime did not advance. I
> guess EEVDF also gets you the same thing but still messes with the CPU
> usage?
> 
> > That said; I'm starting to regret not pushing the EEVDF thing harder
> > back in 2010 when I first wrote it :/
> >
> > > And also CGroups will be even more weird than it already is in such a world,
> > > 2 different containers will not get CPU time distributed properly- say if
> > > tasks in one container sleep a lot and tasks in another container are CPU
> > > bound.
> >
> > Cgroups are an abomination anyway :-) /me runs like hell. But no, I
> > don't actually expect too much trouble there.
> 
> So, with 2 equally weighted containers, if one has a task that sleeps
> 50% of the time, and another has a 100% task, then the sleeper will
> only run 33% of the time? I can see people running containers having a
> problem with that (a customer running one container gets less CPU than
> the other.). Sorry if I missed something.
>

But the 50% sleeper is _asking_ for less CPU.  Doing 50% for each would
mean that when the sleeper task was awake it always ran, always won, to
the exclusion of any one else. (Assuming 1 CPU...)

Cheers,
Phil

> But yeah I do find the whole EEVDF idea interesting but I admit I have
> to research it more.
> 
>  - Joel
> 

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