Re: [PATCH v10 5/9] sched/fair: Take into account latency priority at wakeup

[Vincent Guittot]

On Tue, 21 Feb 2023 at 13:53, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Fri, Jan 13, 2023 at 03:12:30PM +0100, Vincent Guittot wrote:
>
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 6c61bde49152..38decae3e156 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -568,6 +568,8 @@ struct sched_entity {
> >       /* cached value of my_q->h_nr_running */
> >       unsigned long                   runnable_weight;
> >  #endif
> > +     /* preemption offset in ns */
> > +     long                            latency_offset;
>
> I wonder about the type here; does it make sense to have it depend on
> the bitness; that is if s32 is big enough on 32bit then surely it is so
> too on 64bit, and if not, then it should be unconditionally s64.

I mainly wanted to stay aligned with the optimal width of the arch but
32bits is enough

>
>
> > +static void set_latency_offset(struct task_struct *p)
> > +{
> > +     long weight = sched_latency_to_weight[p->latency_prio];
> > +     s64 offset;
> > +
> > +     offset = weight * get_sleep_latency(false);
> > +     offset = div_s64(offset, NICE_LATENCY_WEIGHT_MAX);
> > +     p->se.latency_offset = (long)offset;
> > +}
>
> > +/*
> > + * latency weight for wakeup preemption
> > + */
> > +const int sched_latency_to_weight[40] = {
> > + /* -20 */     -1024,     -973,     -922,      -870,      -819,
> > + /* -15 */      -768,     -717,     -666,      -614,      -563,
> > + /* -10 */      -512,     -461,     -410,      -358,      -307,
> > + /*  -5 */      -256,     -205,     -154,      -102,       -51,
> > + /*   0 */         0,       51,      102,       154,       205,
> > + /*   5 */       256,      307,      358,       410,       461,
> > + /*  10 */       512,      563,      614,       666,       717,
> > + /*  15 */       768,      819,      870,       922,       973,
> > +};
>
> I'm slightly confused by this table, isn't that simply the linear
> function?

Yes, I had in mind to use a nonlinear function at the beginning so the table.

>
> Isn't all that the same as:
>
>         se->se.latency_offset = get_sleep_latency * nice / (NICE_LATENCY_WIDTH/2);
>
> ? The reason we have prio_to_weight[] is because it's an exponential,
> which is a bit more cumbersome to calculate, but surely we can do a
> linear function at runtime.
>
>