linux-kernel - Re: [PATCH v4 4/8] sched/core: Add permission checks for setting the latency

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Message-ID: <CAKfTPtCQTsW2a0fKj8wD5Vmg=ANmY1-SbBQNpqyxJ5FJ67CDgw@mail.gmail.com>
Date:   Wed, 21 Sep 2022 18:11:41 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tim Janik <timj@....org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, parth@...ux.ibm.com,
        qais.yousef@....com, chris.hyser@...cle.com,
        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,
        joshdon@...gle.com
Subject: Re: [PATCH v4 4/8] sched/core: Add permission checks for setting the
 latency_nice value

On Tue, 20 Sept 2022 at 16:56, Vincent Guittot
<vincent.guittot@...aro.org> wrote:
>
> On Tue, 20 Sept 2022 at 12:18, Tim Janik <timj@....org> wrote:
> >
> > Hi.
> >
> > On 19.09.22 14:41, Vincent Guittot wrote:
> > > Hi,
> > >
> > > Thanks you for describing in detail your use case.
> >
> > > Ok, Your explanation makes sense to me especially because we want to
> > > ensure to not provide more cpu time with this latency prio. I'm
> > > curious to see the feedback from others about the reason we want
> > > CAP_SYS_NICE other than following nice priority.
> > >
> > > Side question, Have you tried this patchset (minus this patch) with
> > > your use case ?
> >
> > I have now tested a modified version of the ALSA Test_latency.c program
> > that acquires latency nice as non-root:
> >    https://gist.github.com/tim-janik/88f9df5456b879ecc59da93dc6ce6be1
> >
> > With a busy but not overloaded CPU, the short time latency tests are
> > often better, measured with: ./lnice-latency -p -s 1
> >
> > But the results aren't very reliable with this test. I.e. requesting a
> > latency nice value of -20 reduces the chance for underruns somewhat but
> > doesn't eliminate them (and lnice-latency.c gives up on the first XRUN
>
> It's expected that latency nice can't fix all scheduling latency
> problems. The hard real time constraint can only be ensured with FIFO
> or deadline scheduler
>
> > in the given time period). It might be better to instead count the XRUN
> > occurances over a given time pertiod.
>
> Thanks. I'm going to have a look the test

I have done some tests with your modified ALSA Test_latency.c on my
dev system. I have been able to run lnice-latency -p -s 8 -e -m 128
simultaneously with hackbench -l 20000 -g 2 on my 8 cores arm64
system. The same test with a default latency nice 0 triggers a lot of
XRUN.

Side note, my system doesn't have many RT threads, IRQ or softirq
running . But as explained, latency nice can't do much with those


>
> >
> >
> > --
> > Anklang Free Software DAW
> > https://anklang.testbit.eu/