linux-kernel - Re: [PATCH v10 6/9] sched/fair: Add sched group latency support

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Message-ID: <CAKfTPtBsFn3SW+DLYFNLiaroqFox5sXwBy8zVWZ24F9SKg7FJg@mail.gmail.com>
Date:   Tue, 21 Feb 2023 16:32:37 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, 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,
        cgroups@...r.kernel.org, qyousef@...alina.io,
        chris.hyser@...cle.com, patrick.bellasi@...bug.net,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        tj@...nel.org, 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 v10 6/9] sched/fair: Add sched group latency support

On Tue, 21 Feb 2023 at 16:01, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Jan 13, 2023 at 03:12:31PM +0100, Vincent Guittot wrote:
>
> > +static s64 cpu_latency_nice_read_s64(struct cgroup_subsys_state *css,
> > +                                 struct cftype *cft)
> > +{
> > +     int prio, delta, last_delta = INT_MAX;
> > +     s64 weight;
> > +
> > +     weight = css_tg(css)->latency_offset * NICE_LATENCY_WEIGHT_MAX;
> > +     weight = div_s64(weight, get_sleep_latency(false));
> > +
> > +     /* Find the closest nice value to the current weight */
>
> This comment isn't entirely accurate, since we only have the nice_write
> interface below, this will be an exact match. The thing with weight is
> that we first had the raw weight value interface and then the nice
> interface had to map random values back to a 'nice' value.

Yes, there was a long discussion about the interface and without any
simple raw value to share, we decided to only use latency_nice until
we found a generic metric
>
> Arguably we can simply store the raw nice value in write and print it
> out again here.

Probably, I just wanted to prevent the latency.nice being the main
value saved in cgroup . But I suppose it could be ok to save it
directly

>
> > +     for (prio = 0; prio < ARRAY_SIZE(sched_latency_to_weight); prio++) {
> > +             delta = abs(sched_latency_to_weight[prio] - weight);
> > +             if (delta >= last_delta)
> > +                     break;
> > +             last_delta = delta;
> > +     }
> > +
> > +     return LATENCY_TO_NICE(prio-1);
> > +}
> > +
> > +static int cpu_latency_nice_write_s64(struct cgroup_subsys_state *css,
> > +                                  struct cftype *cft, s64 nice)
> > +{
> > +     s64 latency_offset;
> > +     long weight;
> > +     int idx;
> > +
> > +     if (nice < MIN_LATENCY_NICE || nice > MAX_LATENCY_NICE)
> > +             return -ERANGE;
> > +
> > +     idx = NICE_TO_LATENCY(nice);
> > +     idx = array_index_nospec(idx, LATENCY_NICE_WIDTH);
> > +     weight = sched_latency_to_weight[idx];
> > +
> > +     latency_offset = weight * get_sleep_latency(false);
> > +     latency_offset = div_s64(latency_offset, NICE_LATENCY_WEIGHT_MAX);
> > +
> > +     return sched_group_set_latency(css_tg(css), latency_offset);
> > +}