linux-kernel - Re: [PATCH 0/6] Add latency

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Date:   Mon, 11 Apr 2022 09:26:08 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        chris.hyser@...cle.com, pkondeti@...eaurora.org,
        Valentin.Schneider@....com, patrick.bellasi@...bug.net,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        Wei Wang <wvw@...gle.com>
Subject: Re: [PATCH 0/6] Add latency_nice priority

On Sat, 9 Apr 2022 at 20:10, Qais Yousef <qais.yousef@....com> wrote:
>
> On 04/09/22 13:28, Steven Rostedt wrote:
> > On Sat, 9 Apr 2022 18:08:41 +0100
> > Qais Yousef <qais.yousef@....com> wrote:
> >
> > > One other corner case to consider if you're working on next version is what
> > > should happen when there are multiple tasks of the same priority on the rq. RT
> > > scheduler will push/pull tasks to ensure the task will get to run ASAP if
> > > there's another cpu at lower priority is available. Seems a lot of complexity
> > > to add to CFS, but at the same time if 2 important tasks require low latency
> > > are on the same rq, one of them will suffer without introducing the ability to
> > > migrate one of them where it can get to run sooner.
> >
> > Instead of having the greedy algorithm of the RT push/pull logic, how
> > hard would it be to have the load balancer know of these tasks, and try
> > to keep them on different CPUs? When two are queued on the same CPU,
>
> Oh yeah I didn't think we need to replicate push/pull. Load balancer will need
> to know about it when it moves task so that it avoids placing two of these asks
> on the same cpu.
>
> > could it be possible to just trigger load balancing and let it do the
> > work?
>
> I think the other part will need to be at wake up when we decide the CPU.
>
> If we trigger the load balancing instead then it'd behave like a push/pull?
>
> All these paths are already complex though. So we need to carefully analyze the
> trade-offs. Maybe we don't need to deliver such level of service after all. It
> needs more thinking and experimenting.

I will consider this for v2 but we have to take care of not adding
more latency by trying to find a better CPU. As you said this
additional behavior will need more thinking and experimenting

Vincent

>
> Thanks
>
> --
> Qais Yousef