linux-kernel - Re: [RFC 6/6] sched/fair: Add sched group latency support

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Date:   Tue, 15 Mar 2022 18:07:37 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>, parth@...ux.ibm.com,
        Qais Yousef <qais.yousef@....com>,
        "Hyser,Chris" <chris.hyser@...cle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Valentin Schneider <Valentin.Schneider@....com>,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        Paul Turner <pjt@...gle.com>, pavel@....cz,
        Tejun Heo <tj@...nel.org>,
        Dhaval Giani <dhaval.giani@...cle.com>, qperret@...gle.com,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [RFC 6/6] sched/fair: Add sched group latency support

On Tue, 15 Mar 2022 at 01:59, Josh Don <joshdon@...gle.com> wrote:
>
> On Fri, Mar 11, 2022 at 8:15 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> [snip]
> >
> >  static struct cftype cpu_legacy_files[] = {
> > @@ -10649,6 +10673,11 @@ static struct cftype cpu_legacy_files[] = {
> >                 .read_s64 = cpu_idle_read_s64,
> >                 .write_s64 = cpu_idle_write_s64,
> >         },
> > +       {
> > +               .name = "latency",
> > +               .read_s64 = cpu_latency_read_s64,
> > +               .write_s64 = cpu_latency_write_s64,
> > +       },
> >  #endif
> >  #ifdef CONFIG_CFS_BANDWIDTH
> >         {
> > @@ -10866,6 +10895,18 @@ static struct cftype cpu_files[] = {
> >                 .read_s64 = cpu_idle_read_s64,
> >                 .write_s64 = cpu_idle_write_s64,
> >         },
> > +       {
> > +               .name = "latency",
> > +               .flags = CFTYPE_NOT_ON_ROOT,
> > +               .read_s64 = cpu_latency_read_s64,
> > +               .write_s64 = cpu_latency_write_s64,
> > +       },
> > +       {
> > +               .name = "latency.nice",
> > +               .flags = CFTYPE_NOT_ON_ROOT,
> > +               .read_s64 = cpu_latency_nice_read_s64,
> > +               .write_s64 = cpu_latency_nice_write_s64,
> > +       },
>
> Something I considered when adding cpu.idle was that negative values
> could be used to indicate increasing latency sensitivity. Folding the
> above latency property into cpu.idle could help consolidate the
> "latency" behavior, especially given that it shouldn't really be
> possible to configure an entity as both latency sensitive and idle.

The range of latency nice is [-19:20] and it doesn't touch on the
weight whereas sched_idle behavior impacts both the shares and the
preemption so I was afraid of possible confusion with what latency
nice is doing