Date:   Fri, 24 Feb 2023 20:29:19 +0100
From:   Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org, parth@linux.ibm.com, tj@kernel.org,
        lizefan.x@bytedance.com, hannes@cmpxchg.org,
        cgroups@vger.kernel.org, corbet@lwn.net, linux-doc@vger.kernel.org,
        qyousef@layalina.io, chris.hyser@oracle.com,
        patrick.bellasi@matbug.net, David.Laight@aculab.com,
        pjt@google.com, pavel@ucw.cz, qperret@google.com,
        tim.c.chen@linux.intel.com, joshdon@google.com, timj@gnu.org,
        kprateek.nayak@amd.com, yu.c.chen@intel.com,
        youssefesmat@chromium.org, joel@joelfernandes.org
Subject: Re: [PATCH v12 6/8] sched/fair: Add sched group latency support
Message-ID: <20230224192919.d4fcde3dwh7betvm@blackpad>
References: <20230224093454.956298-1-vincent.guittot@linaro.org>
 <20230224093454.956298-7-vincent.guittot@linaro.org>
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Hello Vincent.

On Fri, Feb 24, 2023 at 10:34:52AM +0100, Vincent Guittot <vincent.guittot@linaro.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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