Re: [PATCH v4 1/3] sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL sched_domain flag

From: Vincent Guittot <vincent.guittot@linaro.org>
To: Beata Michalska <beata.michalska@arm.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	"corbet@lwn.net" <corbet@lwn.net>,
	Randy Dunlap <rdunlap@infradead.org>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v4 1/3] sched/core: Introduce SD_ASYM_CPUCAPACITY_FULL sched_domain flag
Date: Tue, 18 May 2021 16:53:09 +0200	[thread overview]
Message-ID: <CAKfTPtAk8pQfpN7FrBqdOiSz2Ncby4ozXOgQvT_QZMX67-FRKA@mail.gmail.com> (raw)
In-Reply-To: <20210518142746.GA3993@e120325.cambridge.arm.com>

On Tue, 18 May 2021 at 16:27, Beata Michalska <beata.michalska@arm.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@arm.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.

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 ?

>
> ---
> 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@arm.com>
> > > Reviewed-by: Valentin Schneider <valentin.schneider@arm.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
> > >