Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with sched_domain_shared

[Michael Neuling <mikey@neuling.org>]

On Wed, 2016-05-11 at 20:24 +0200, Peter Zijlstra wrote:
> On Wed, May 11, 2016 at 02:33:45PM +0200, Peter Zijlstra wrote:
> > 
> > Hmm, PPC folks; what does your topology look like?
> > 
> > Currently your sched_domain_topology, as per arch/powerpc/kernel/smp.c
> > seems to suggest your cores do not share cache at all.
> > 
> > https://en.wikipedia.org/wiki/POWER7 seems to agree and states
> > 
> >   "4 MB L3 cache per C1 core"
> > 
> > And http://www-03.ibm.com/systems/resources/systems_power_software_i_pe
> > rfmgmt_underthehood.pdf
> > also explicitly draws pictures with the L3 per core.
> > 
> > _however_, that same document describes L3 inter-core fill and lateral
> > cast-out, which sounds like the L3s work together to form a node wide
> > caching system.
> > 
> > Do we want to model this co-operative L3 slices thing as a sort of
> > node-wide LLC for the purpose of the scheduler ?
> Going back a generation; Power6 seems to have a shared L3 (off package)
> between the two cores on the package. The current topology does not
> reflect that at all.
> 
> And going forward a generation; Power8 seems to share the per-core
> (chiplet) L3 amonst all cores (chiplets) + is has the centaur (memory
> controller) 16M L4.

Yep, L1/L2/L3 is per core on POWER8 and POWER7.  POWER6 and POWER5 (both
dual core chips) had a shared off chip cache

The POWER8 L4 is really a bit different as it's out in the memory
controller.  It's more of a memory DIMM buffer as it can only cache data
associated with the physical addresses on those DIMMS.

> So it seems the current topology setup is not describing these chips
> very well. Also note that the arch topology code can runtime select a
> topology, so you could make that topo setup micro-arch specific.

We are planning on making some topology changes for the upcoming P9 which
will share L2/L3 amongst pairs of cores (24 cores per chip).

FWIW our P9 upstreaming is still in it's infancy since P9 is not released
yet.

Mike