linux-kernel - Re: [PATCH 0/2] sched/eevdf: sched_attr::sched

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Message-ID: <c30367d351c821b13c2bdd9d680388d244fa74c5.camel@gmx.de>
Date:   Wed, 20 Sep 2023 06:02:17 +0200
From:   Mike Galbraith <efault@....de>
To:     Qais Yousef <qyousef@...alina.io>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, vincent.guittot@...aro.org,
        juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, corbet@....net, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, 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,
        tglx@...utronix.de, daniel.m.jordan@...cle.com
Subject: Re: [PATCH 0/2] sched/eevdf: sched_attr::sched_runtime slice hint

On Tue, 2023-09-19 at 22:08 +0100, Qais Yousef wrote:
> On 09/18/23 05:43, Mike Galbraith wrote:
> > On Sat, 2023-09-16 at 22:33 +0100, Qais Yousef wrote:
> > >
> > > Example of conflicting requirements that come across frequently:
> > >
> > >         1. Improve wake up latency without for SCHED_OTHER. Many tasks
> > >            end up using SCHED_FIFO/SCHED_RR to compensate for this
> > >            shortcoming. RT tasks lack power management and fairness and
> > >            can be hard and error prone to use correctly and portably.
> >
> > This bit appears to be dealt with about as nicely as it can be in a
> > fair class by the latency nice patch set, and deals with both
> > individual tasks and groups thereof, ie has cgroups support.
>
> AFAIU the latency_nice is no longer going forward. But I could be mistaken.

Effectively it is, both making the same request under the hood, the
difference being trade negotiation idiom.

I took both to try out for no particularly good reason.  The only thing
silly looking in the result is one clipping at OMG the other at OMFG.

> > All three of those make my eyebrows twitch mightily even in their not
> > well defined form: any notion of applying badges to identify groups of
> > tasks would constitute creation of yet another cgroups.
>
> cgroups require root privilege. And it is intended for sysadmins to split
> system resources between apps. It doesn't help an app to describe the
> relationship between its tasks. Nor any requirements for them to do their job
> properly. But rather impose something on them regardless of what they want.

The whys and wherefores are clear.  I suspect that addition of another
task group interface with conflicting scheduling parameters, policies,
hopes and/or prayers to be dealt with at each and every level of the
existing hierarchy is going to be hard to sell, but who knows, maybe
that skeleton looks more attractive to maintainers than it does to me.
I suppose we'll find out once you hang some meat on it.

	-Mike