linux-kernel - Re: [PATCH v2 0/7] Add latency

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Date:   Fri, 13 May 2022 16:29:55 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "vschneid@...hat.com" <vschneid@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "parth@...ux.ibm.com" <parth@...ux.ibm.com>,
        "qais.yousef@....com" <qais.yousef@....com>,
        "chris.hyser@...cle.com" <chris.hyser@...cle.com>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "patrick.bellasi@...bug.net" <patrick.bellasi@...bug.net>,
        "pjt@...gle.com" <pjt@...gle.com>, "pavel@....cz" <pavel@....cz>,
        "tj@...nel.org" <tj@...nel.org>,
        "qperret@...gle.com" <qperret@...gle.com>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "joshdon@...gle.com" <joshdon@...gle.com>
Subject: Re: [PATCH v2 0/7] Add latency_nice priority

On Fri, 13 May 2022 at 09:13, David Laight <David.Laight@...lab.com> wrote:
>
> From: Vincent Guittot
> > Sent: 12 May 2022 17:35
> >
> > This patchset restarts the work about adding a latency nice priority to
> > describe the latency tolerance of cfs tasks.
> >
> > The patches [1-4] have been done by Parth:
> > https://lore.kernel.org/lkml/20200228090755.22829-1-parth@linux.ibm.com/
> >
> > I have just rebased and moved the set of latency priority outside the
> > priority update. I have removed the reviewed tag because the patches
> > are 2 years old.
> >
> > The patches [5-7] use latency nice priority to decide if a cfs task can
> > preempt the current running task. Patch 5 gives some tests results with
> > cyclictests and hackbench to highlight the benefit of latency nice
> > priority for short interactive task or long intensive tasks.
>
> I'd have thought the best way to reduce latency would be to look
> harder for an idle cpu before trying to preempt the current task.

Although it's a good policy, this is not always true:
- The wakeup latency of an idle CPU can be significant for deep idle
state (several ms)
- The cost of looking for an idle CPU can be higher than preempting
current thread

>
> By far the worst case latency for a cfs task is waking it from an
> rt task.
> AFAICT the cpu selection algorithm is something like:
> 1) if the cpu it last ran on is idle, schedule it there.
> 2) if one of a small subset of cpu is idle run it there.

yes the LLC

> 3) schedule on the current cpu after the rt thread exits.

when prev and current cpu are differents, we start to select which one
will be the starting point for looking for an idle cpu

>
> Then there is the additional behaviour:
> An rt thread (almost) always starts on the cpu it ran on last,
> any running cfs thread is put on the q for that cpu.

and might even have to wait for other cfs threads to run 1st which is
where this patch want to improve in priority

>
> This makes it very difficult to start a background cfs thread
> from an active rt thread.
> Quite often it won't migrate to an idle cpu until the timer
> tick scheduler rebalance happens.

so you are speaking about idle cpu out of the LLC
But then, this cfs task should try to wakeup 1st on the new cpu and
not on the RT one

>
> I think the search for an idle cpu is also limited when woken
> by a cfs thread.
>
>         David
>
> -
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