linux-kernel - Re: [PATCH v12 6/8] sched/fair: Add sched group latency support

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Message-ID: <20230224192919.d4fcde3dwh7betvm@blackpad>
Date:   Fri, 24 Feb 2023 20:29:19 +0100
From:   Michal Koutný <mkoutny@...e.com>
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, tj@...nel.org,
        lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, corbet@....net, linux-doc@...r.kernel.org,
        qyousef@...alina.io, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        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, joel@...lfernandes.org
Subject: Re: [PATCH v12 6/8] sched/fair: Add sched group latency support

Hello Vincent.

On Fri, Feb 24, 2023 at 10:34:52AM +0100, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> +  cpu.latency.nice
> +	A read-write single value file which exists on non-root
> +	cgroups.  The default is "0".
> +
> +	The nice value is in the range [-20, 19].
> +
> +	This interface file allows reading and setting latency using the
> +	same values used by sched_setattr(2). The latency_nice of a group is
> +	used to limit the impact of the latency_nice of a task outside the
> +	group.

IIUC, the latency priority is taken into account when deciding between
entitites at the same level (as in pick_next_entity() or
check_preempt_wake()/find_matchig_se()).

So this group attribute is relevant in context of siblings (i.e. like
cpu.weight ~ bandwidth priority)?

I'm thus confused when it's referred to as a limit (in vertical sense).
You somewhat imply that in [1]:

> Regarding the behavior, the rule remains the same that a sched_entity
> attached to a cgroup will not get more (latency in this case) than
> what has been set for the group entity.

But I don't see where such a constraint would be implemented in the
code. (My cursory understanding above tends to horizontal comparisons.)

Could you please hint me which is right?

Thanks,
Michal

[1] https://lore.kernel.org/r/CAKfTPtDu=c-psGnHkoWSPRWoh1Z0VBBfsN++g+krv4B1SJmFjg@mail.gmail.com/


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