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

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Date:   Wed, 30 Nov 2022 14:42:37 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     mingo@...hat.com, peterz@...radead.org, 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,
        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
Subject: Re: [PATCH 5/9] sched/fair: Take into account latency priority at wakeup

On Wed, 30 Nov 2022 at 04:10, Joel Fernandes <joel@...lfernandes.org> wrote:
>
> Hi Vincent,
>
> On Tue, Nov 29, 2022 at 5:21 PM Vincent Guittot
> <vincent.guittot@...aro.org> 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@...lfernandes.org>
>
> I am excited about your series, the CFS latency issues have been
> thorny. This feels like a step forward in the right direction. Cheers,

Thanks
Vincent

>
>  - Joel