linux-kernel - Re: [RFC/PATCH] powerpc/smp: Add SD_SHARE_PKG

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Message-ID: <CAKfTPtDX-p=gWAVgYzLNCNuQ2e=QP2pTeMs=BmNBo31fpGKxrg@mail.gmail.com>
Date:   Mon, 12 Apr 2021 14:21:47 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Rik van Riel <riel@...riel.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Nicholas Piggin <npiggin@...il.com>,
        Anton Blanchard <anton@...abs.org>,
        Parth Shah <parth@...ux.ibm.com>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC/PATCH] powerpc/smp: Add SD_SHARE_PKG_RESOURCES flag to MC sched-domain

On Mon, 12 Apr 2021 at 11:37, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Mon, Apr 12, 2021 at 11:54:36AM +0530, Srikar Dronamraju wrote:
> > * Gautham R. Shenoy <ego@...ux.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