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

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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_perfmgmt_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.

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.

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[RFC][PATCH 0/7] sched: select_idle_siblings rewrite Peter Zijlstra <peterz@infradead.org> - 2016-05-09 13:00 +0200
  [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-09 13:00 +0200
    Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Matt Fleming <matt@codeblueprint.co.uk> - 2016-05-11 14:00 +0200
      Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-11 14:40 +0200
        Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-11 20:20 +0200
        Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-11 20:30 +0200
          Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Michael Neuling <mikey@neuling.org> - 2016-05-12 04:10 +0200
            Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-12 07:10 +0200
              Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Michael Neuling <mikey@neuling.org> - 2016-05-12 13:10 +0200
                Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-12 13:40 +0200
      Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-11 19:40 +0200
        Re: [RFC][PATCH 4/7] sched: Replace sd_busy/nr_busy_cpus with  sched_domain_shared Matt Fleming <matt@codeblueprint.co.uk> - 2016-05-11 20:10 +0200
  [RFC][PATCH 1/7] sched: Remove unused @cpu argument from destroy_sched_domain*() Peter Zijlstra <peterz@infradead.org> - 2016-05-09 13:00 +0200
  [RFC][PATCH 3/7] sched: Introduce struct sched_domain_shared Peter Zijlstra <peterz@infradead.org> - 2016-05-09 13:00 +0200
  [RFC][PATCH 6/7] sched: Optimize SCHED_SMT Peter Zijlstra <peterz@infradead.org> - 2016-05-09 13:00 +0200
  Re: [RFC][PATCH 0/7] sched: select_idle_siblings rewrite Chris Mason <clm@fb.com> - 2016-05-10 03:00 +0200
  Re: [RFC][PATCH 0/7] sched: select_idle_siblings rewrite Chris Mason <clm@fb.com> - 2016-05-11 16:30 +0200

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