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

[Valentin Schneider <valentin.schneider@arm.com>]

On 12/04/21 10:37, Mel Gorman 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?
>>
>
> 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
> allows within the node with the LLC CPUs masked out.

I think that's actually a decent approach. Tying SD_SHARE_PKG_RESOURCES to
something other than pure cache topology in a generic manner is tough (as
it relies on murky, ill-defined hardware fabric properties).

Last I tried thinking about that, I stopped at having a core-to-core
latency matrix, building domains off of that, and having some knob
specifying the highest distance value below which we'd set
SD_SHARE_PKG_RESOURCES. There's a few things I 'hate' about that; for one
it makes cpus_share_cache() somewhat questionable.

> 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.
>
> --
> Mel Gorman
> SUSE Labs