[RFC/PATCH] powerpc/smp: Add SD_SHARE_PKG_RESOURCES flag to MC sched-domain

[Vincent Guittot]

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