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

[Joel Fernandes]

Hi Vincent,

On Tue, Nov 29, 2022 at 5:21 PM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
[...]
> > >>> }
> > >>>
> > >>> /*
> > >>> @@ -7544,7 +7558,7 @@ static int __sched_setscheduler(struct task_struct *p,
> > >>>                if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)
> > >>>                        goto change;
> > >>>                if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE &&
> > >>> -                   attr->sched_latency_nice != p->latency_nice)
> > >>> +                   attr->sched_latency_nice != LATENCY_TO_NICE(p->latency_prio))
> > >>>                        goto change;
> > >>>
> > >>>                p->sched_reset_on_fork = reset_on_fork;
> > >>> @@ -8085,7 +8099,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> > >>>        get_params(p, &kattr);
> > >>>        kattr.sched_flags &= SCHED_FLAG_ALL;
> > >>>
> > >>> -       kattr.sched_latency_nice = p->latency_nice;
> > >>> +       kattr.sched_latency_nice = LATENCY_TO_NICE(p->latency_prio);
> > >>>
> > >>> #ifdef CONFIG_UCLAMP_TASK
> > >>>        /*
> > >>> @@ -11294,6 +11308,20 @@ const u32 sched_prio_to_wmult[40] = {
> > >>>  /*  15 */ 119304647, 148102320, 186737708, 238609294, 286331153,
> > >>> };
> > >>>
> > >>> +/*
> > >>> + * 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,
> > >>> +};
> > >>> +
> > >>
> > >> The table is linear. You could approximate this as: weight = nice * 51
> > >> since it is a linear scale and do the conversion in place.
> > >>
> > >> Or, since the only place you are using the latency_to_weight is in
> > >> set_latency_offset(), can we drop the sched_latency_to_weight array
> > >> and simplify as follows?
> > >
> > > It's also used in cgroup patch and keeps a coherency between
> > > nice/weight an latency_nice/offset so I prefer
> >
> > I dont think it’s a valid comparison as nice/weight conversion are non linear and over there a table makes sense: weight = 1024 / 1.25 ^ nice
> >
> > > keeping current
> > > implementation
> >
> > I could be missing something, but, since its a linear scale, why does cgroup need weight at all? Just store nice directly. Why would that not work?
> >
> > In the end the TG and SE has the latency offset in the struct, that is all you care about. All the conversion back and forth is unnecessary, as it is a linear scale and just increases LOC and takes more memory to store linear arrays.
> >
> > Again I could be missing something and I will try to play with your series and see if I can show you what I mean (or convince myself it’s needed).
>
> I get what you mean but I think that having an array gives latitude to
> adjust this internal offset mapping at a minimum cost of a const array

Ok that makes sense. If you feel like there might be updates in the
future to this mapping array (like changing the constants as you
mentioned), then I am Ok with us keeping it.

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

I am excited about your series, the CFS latency issues have been
thorny. This feels like a step forward in the right direction. Cheers,

 - Joel