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

[Michal Koutný]

Hello Vincent.

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

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