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

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Date:   Tue, 20 Sep 2022 12:18:38 +0200
From:   Tim Janik <timj@....org>
To:     Vincent Guittot <vincent.guittot@...aro.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

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 
in the given time period). It might be better to instead count the XRUN 
occurances over a given time pertiod.


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