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From: Vincent Guittot <vincent.guittot@linaro.org>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>,
	Michael Neuling <mikey@neuling.org>,
	Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
	Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
	Rik van Riel <riel@surriel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Parth Shah <parth@linux.ibm.com>,
	linuxppc-dev@lists.ozlabs.org,
	Valentin Schneider <valentin.schneider@arm.com>
Subject: Re: [RFC/PATCH] powerpc/smp: Add SD_SHARE_PKG_RESOURCES flag to MC sched-domain
Date: Mon, 12 Apr 2021 14:21:47 +0200
Message-ID: <CAKfTPtDX-p=gWAVgYzLNCNuQ2e=QP2pTeMs=BmNBo31fpGKxrg@mail.gmail.com> (raw)
In-Reply-To: <20210412093722.GS3697@techsingularity.net>

On Mon, 12 Apr 2021 at 11:37, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Mon, Apr 12, 2021 at 11:54:36AM +0530, Srikar Dronamraju wrote:
> > * Gautham R. Shenoy <ego@linux.vnet.ibm.com> [2021-04-02 11:07:54]:
> >
> > >
> > > To remedy this, this patch proposes that the LLC be moved to the MC
> > > level which is a group of cores in one half of the chip.
> > >
> > >       SMT (SMT4) --> MC (Hemisphere)[LLC] --> DIE
> > >
> >
> > I think marking Hemisphere as a LLC in a P10 scenario is a good idea.
> >
> > > While there is no cache being shared at this level, this is still the
> > > level where some amount of cache-snooping takes place and it is
> > > relatively faster to access the data from the caches of the cores
> > > within this domain. With this change, we no longer see regressions on
> > > P10 for applications which require single threaded performance.
> >
> > Peter, Valentin, Vincent, Mel, etal
> >
> > On architectures where we have multiple levels of cache access latencies
> > within a DIE, (For example: one within the current LLC or SMT core and the
> > other at MC or Hemisphere, and finally across hemispheres), do you have any
> > suggestions on how we could handle the same in the core scheduler?

I would say that SD_SHARE_PKG_RESOURCES is there for that and doesn't
only rely on cache

> >
>
> Minimally I think it would be worth detecting when there are multiple
> LLCs per node and detecting that in generic code as a static branch. In
> select_idle_cpu, consider taking two passes -- first on the LLC domain
> and if no idle CPU is found then taking a second pass if the search depth

We have done a lot of changes to reduce and optimize the fast path and
I don't think re adding another layer  in the fast path makes sense as
you will end up unrolling the for_each_domain behind some
static_banches.

SD_SHARE_PKG_RESOURCES should be set to the last level where we can
efficiently move task between CPUs at wakeup

> allows within the node with the LLC CPUs masked out. While there would be
> a latency hit because cache is not shared, it would still be a CPU local
> to memory that is idle. That would potentially be beneficial on Zen*
> as well without having to introduce new domains in the topology hierarchy.

What is the current sched_domain topology description for zen ?

>
> --
> Mel Gorman
> SUSE Labs

  parent reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-02  5:37 Gautham R. Shenoy
2021-04-02  7:36 ` Gautham R Shenoy
2021-04-12  6:24 ` Srikar Dronamraju
2021-04-12  9:37   ` Mel Gorman
2021-04-12 10:06     ` Valentin Schneider
2021-04-12 10:48       ` Mel Gorman
2021-04-19  6:14         ` Gautham R Shenoy
2021-04-12 12:21     ` Vincent Guittot [this message]
2021-04-12 15:24       ` Mel Gorman
2021-04-12 16:33         ` Michal Suchánek
2021-04-14  7:02           ` Gautham R Shenoy
2021-04-13  7:10         ` Vincent Guittot
2021-04-14  7:00         ` Gautham R Shenoy

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