linux-kernel - Re: [PATCH v4 1/3] sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL sched

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Message-ID: <CAKfTPtAYjzMfj3Uh=xjAeiOZtkb5iyPA72+ow+RXYtQq0mGNDA@mail.gmail.com>
Date:   Tue, 18 May 2021 17:28:11 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Beata Michalska <beata.michalska@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "corbet@....net" <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v4 1/3] sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL
 sched_domain flag

On Tue, 18 May 2021 at 17:09, Beata Michalska <beata.michalska@....com> wrote:
>
> On Tue, May 18, 2021 at 04:53:09PM +0200, Vincent Guittot wrote:
> > On Tue, 18 May 2021 at 16:27, Beata Michalska <beata.michalska@....com> wrote:
> > >
> > > On Tue, May 18, 2021 at 03:39:27PM +0200, Vincent Guittot wrote:
> > > > On Mon, 17 May 2021 at 10:24, Beata Michalska <beata.michalska@....com> wrote:
> > > > >
> > > > > Introducing new, complementary to SD_ASYM_CPUCAPACITY, sched_domain
> > > > > topology flag, to distinguish between shed_domains where any CPU
> > > > > capacity asymmetry is detected (SD_ASYM_CPUCAPACITY) and ones where
> > > > > a full range of CPU capacities is visible to all domain members
> > > > > (SD_ASYM_CPUCAPACITY_FULL).
> > > >
> > > > I'm not sure about what you want to detect:
> > > >
> > > > Is it a sched_domain level with a full range of cpu capacity, i.e.
> > > > with at least 1 min capacity and 1 max capacity ?
> > > > or do you want to get at least 1 cpu of each capacity ?
> > > That would be at least one CPU of each available capacity within given domain,
> > > so full -set- of available capacities within a domain.
> >
> > Would be good to add the precision.
> Will do.
> >
> > Although I'm not sure if that's the best policy compared to only
> > getting the range which would be far simpler to implement.
> > Do you have some topology example ?
>
> An example from second patch from the series:
>
> DIE      [                                ]
> MC       [                       ][       ]
>
> CPU       [0] [1] [2] [3] [4] [5]  [6] [7]
> Capacity  |.....| |.....| |.....|  |.....|
>              L       M       B        B

The one above , which is described in your patchset, works with the range policy

>
> Where:
>  arch_scale_cpu_capacity(L) = 512
>  arch_scale_cpu_capacity(M) = 871
>  arch_scale_cpu_capacity(B) = 1024
>
> which could also look like:
>
> DIE      [                                        ]
> MC       [                       ][               ]
>
> CPU       [0] [1] [2] [3] [4] [5]  [6] [7] [8] [9]
> Capacity  |.....| |.....| |.....|  |.....| |.....|
>              L       M       B        L       B

I know that that HW guys can come with crazy idea but they would
probably add M  instead of L with B in the 2nd cluster as a boost of
performance at the cost of powering up another "cluster" in which case
the range policy works as well

>
> Considering only range would mean loosing the 2 (M) CPUs out of sight
> for feec in some cases.

Is it realistic ? Considering all the code and complexity added by
patch 2, will we really use it at the end ?

Regards,
Vincent
>
> ---
> BR.
> B
> >
> >
> >
> >
> >
> >
> > >
> > > ---
> > > BR
> > > B.
> > > >
> > > >
> > > > >
> > > > > With the distinction between full and partial CPU capacity asymmetry,
> > > > > brought in by the newly introduced flag, the scope of the original
> > > > > SD_ASYM_CPUCAPACITY flag gets shifted, still maintaining the existing
> > > > > behaviour when one is detected on a given sched domain, allowing
> > > > > misfit migrations within sched domains that do not observe full range
> > > > > of CPU capacities but still do have members with different capacity
> > > > > values. It loses though it's meaning when it comes to the lowest CPU
> > > > > asymmetry sched_domain level per-cpu pointer, which is to be now
> > > > > denoted by SD_ASYM_CPUCAPACITY_FULL flag.
> > > > >
> > > > > Signed-off-by: Beata Michalska <beata.michalska@....com>
> > > > > Reviewed-by: Valentin Schneider <valentin.schneider@....com>
> > > > > ---
> > > > >  include/linux/sched/sd_flags.h | 10 ++++++++++
> > > > >  1 file changed, 10 insertions(+)
> > > > >
> > > > > diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> > > > > index 34b21e9..57bde66 100644
> > > > > --- a/include/linux/sched/sd_flags.h
> > > > > +++ b/include/linux/sched/sd_flags.h
> > > > > @@ -91,6 +91,16 @@ SD_FLAG(SD_WAKE_AFFINE, SDF_SHARED_CHILD)
> > > > >  SD_FLAG(SD_ASYM_CPUCAPACITY, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> > > > >
> > > > >  /*
> > > > > + * Domain members have different CPU capacities spanning all unique CPU
> > > > > + * capacity values.
> > > > > + *
> > > > > + * SHARED_PARENT: Set from the topmost domain down to the first domain where
> > > > > + *               all available CPU capacities are visible
> > > > > + * NEEDS_GROUPS: Per-CPU capacity is asymmetric between groups.
> > > > > + */
> > > > > +SD_FLAG(SD_ASYM_CPUCAPACITY_FULL, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
> > > > > +
> > > > > +/*
> > > > >   * Domain members share CPU capacity (i.e. SMT)
> > > > >   *
> > > > >   * SHARED_CHILD: Set from the base domain up until spanned CPUs no longer share
> > > > > --
> > > > > 2.7.4
> > > > >