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

From: Mel Gorman <[email-masked]>
Date: Mon, 12 Apr 2021 16:24:44 +0100

On Mon, Apr 12, 2021 at 02:21:47PM +0200, Vincent Guittot wrote:
> > > 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
> 

>From topology.c

	SD_SHARE_PKG_RESOURCES - describes shared caches

I'm guessing here because I am not familiar with power10 but the central
problem appears to be when to prefer selecting a CPU sharing L2 or L3
cache and the core assumes the last-level-cache is the only relevant one.

For this patch, I wondered if setting SD_SHARE_PKG_RESOURCES would have
unintended consequences for load balancing because load within a die may
not be spread between SMT4 domains if SD_SHARE_PKG_RESOURCES was set at
the MC level.

> >
> > 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.
> 

Searching the node would only happen if a) there was enough search depth
left and b) there were no idle CPUs at the LLC level. As no new domain
is added, it's not clear to me why for_each_domain would change.

But still, your comment reminded me that different architectures have
different requirements

Power 10 appears to prefer CPU selection sharing L2 cache but desires
	spillover to L3 when selecting and idle CPU.

X86 varies, it might want the Power10 approach for some families and prefer
	L3 spilling over to a CPU on the same node in others.

S390 cares about something called books and drawers although I've no
	what it means as such and whether it has any preferences on
	search order.

ARM has similar requirements again according to "scheduler: expose the
	topology of clusters and add cluster scheduler" and that one *does*
	add another domain.

I had forgotten about the ARM patches but remembered that they were
interesting because they potentially help the Zen situation but I didn't
get the chance to review them before they fell off my radar again. About
all I recall is that I thought the "cluster" terminology was vague.

The only commonality I thought might exist is that architectures may
like to define what the first domain to search for an idle CPU and a
second domain. Alternatively, architectures could specify a domain to
search primarily but also search the next domain in the hierarchy if
search depth permits. The default would be the existing behaviour --
search CPUs sharing a last-level-cache.

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

The definition of "efficiently" varies. Moving tasks between CPUs sharing
a cache is most efficient but moving the task to a CPU that at least has
local memory channels is a reasonable option if there are no idle CPUs
sharing cache and preferable to stacking.

> > 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 ?
> 

The cache and NUMA topologies differ slightly between each generation
of Zen. The common pattern is that a single NUMA node can have multiple
L3 caches and at one point I thought it might be reasonable to allow
spillover to select a local idle CPU instead of stacking multiple tasks
on a CPU sharing cache. I never got as far as thinking how it could be
done in a way that multiple architectures would be happy with.

-- 
Mel Gorman
SUSE Labs