linux-kernel - Re: [PATCH v3] sched/fair: unlink misfit task from cpu overutilized

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Message-ID: <20230118164820.r37yfigmwwdske4r@airbuntu>
Date:   Wed, 18 Jan 2023 16:48:20 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...nel.org,
        peterz@...radead.org, rafael@...nel.org, viresh.kumar@...aro.org,
        vschneid@...hat.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, lukasz.luba@....com, wvw@...gle.com,
        xuewen.yan94@...il.com, han.lin@...iatek.com,
        Jonathan.JMChen@...iatek.com
Subject: Re: [PATCH v3] sched/fair: unlink misfit task from cpu overutilized

On 01/18/23 09:15, Vincent Guittot wrote:
> On Mon, 16 Jan 2023 at 15:56, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >
> > On 16/01/2023 12:23, Vincent Guittot wrote:
> > > On Mon, 16 Jan 2023 at 10:07, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> > >>
> > >> On 13/01/2023 14:40, Vincent Guittot wrote:
> >
> > [...]
> >
> > >>> @@ -6132,6 +6135,7 @@ static inline bool cpu_overutilized(int cpu)
> > >>>       unsigned long rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
> > >>>       unsigned long rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);
> > >>>
> > >>> +     /* Return true only if the utlization doesn't fit its capacity */
> > >>
> > >> s/utlization/utilization
> > >> s/its/CPU ?
> > >>
> > >>>       return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu);
> > >>>  }
> > >>
> > >> cpu_overutilized() is the only place where we now only test for
> > >> !util_fits_cpu(). The new comment says we only care about utilization
> > >> not fitting CPU capacity.
> > >>
> > >> Does this mean the rq uclamp values are not important here and we could
> > >> go back to use fits_capacity()?
> > >>
> > >> Not sure since util_fits_cpu() is still coded differently:
> > >
> > > uclamp_min is not important but uclamp_max still cap the utilization
> >
> > OK, makes sense.
> >
> > I.e. we could pass in `rq_util_min = 0` to avoid fetching it
> > unnecessary? In case `fits == 1` before the uclamp_min condition in
> > util_fits_cpu() it doesn't matter if we switch to return `-1` when
> > called from cpu_overutilized(). Detail though ...
> 
> One comment from Qais was to minimize knowledge outside
> util_fits_cpu() that's why I pass both uclamp_min and uclamp_max.
> 
> >
> > [...]
> >
> > >>> @@ -6940,12 +6945,28 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
> > >>>
> > >>>               if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu))
> > >>>                       continue;
> > >>> -             if (util_fits_cpu(task_util, util_min, util_max, cpu))
> > >>> +
> > >>> +             fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> > >>> +
> > >>> +             /* This CPU fits with all capacity and performance requirements */
> > >>
> > >> In task_fits_cpu() `utilization and performance (better uclamp)
> > >> requirements` term was used. I assume it's the same thing here?
> > >>
> > >>> +             if (fits > 0)
> > >>>                       return cpu;
> > >>> +             /*
> > >>> +              * Only the min performance (i.e. uclamp_min) doesn't fit. Look
> > >>> +              * for the CPU with highest performance capacity.
> > >>                                             ^^^^^^^^^^^^^^^^^^^^
> > >>
> > >> Do we use a new CPU capacity value `performance capacity (1)` here?
> > >>
> > >> Which I guess is `capacity_orig_of(cpu) - thermal_load_avg(cpu_rq(cpu)`.
> > >>
> > >> I'm asking since util_fits_cpu() still uses: `capacity_orig_thermal (2)
> > >> = capacity_orig - arch_scale_thermal_pressure()` when checking whether
> > >> to return -1. Shouldn't (1) and (2) be the same?
> > >
> > > I'm all in favor of both being capacity_orig_of(cpu) -
> > > thermal_load_avg(cpu_rq(cpu) like the capacity inversion detection
> >
> > I think we need a handy name for this new capacity value, which seems to
> > be `capacity_orig - capacity reduced by thermal`. And we should either
> > use `thermal_load_avg` or `thermal pressure` for the latter part. And
> > then we should use this consistently in all these places:
> > util_fits_cpu(), feec(), sic().
> 
> Ok, let me change this everywhere

I'm not keen on this :-/

Changing this everywhere could have implications beyond our simple capabilities
of testing now :(

Current choice (in util_fits_cpu()) was based on a direct feedback from Xuewen.
I think we should discuss how we can improve the situation instead rather than
worry about consistency. I don't think we can be consistent without doing some
improvements on thermal pressure response time.

A separate proposal patch to invoke some testing and discussion is fine by me.

Better keep it a separate work item please?


Cheers

--
Qais Yousef