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

[Vincent Guittot]

On Tue, 21 Feb 2023 at 16:01, Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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);
> > +}