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Date:   Wed, 19 Jun 2019 22:34:37 +0100
From:   Matt Fleming <matt@codeblueprint.co.uk>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@amd.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH] sched/topology: Improve load balancing on AMD EPYC
Message-ID: <20190619213437.GA6909@codeblueprint.co.uk>
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On Tue, 18 Jun, at 02:33:18PM, Peter Zijlstra wrote:
> On Tue, Jun 18, 2019 at 11:43:19AM +0100, Matt Fleming wrote:
> > This works for me under all my tests. Thoughts?
> > 
> > --->8---
> > 
> > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> > index 80a405c2048a..4db4e9e7654b 100644
> > --- a/arch/x86/kernel/cpu/amd.c
> > +++ b/arch/x86/kernel/cpu/amd.c
> > @@ -8,6 +8,7 @@
> >  #include <linux/sched.h>
> >  #include <linux/sched/clock.h>
> >  #include <linux/random.h>
> > +#include <linux/topology.h>
> >  #include <asm/processor.h>
> >  #include <asm/apic.h>
> >  #include <asm/cacheinfo.h>
> > @@ -824,6 +825,8 @@ static void init_amd_zn(struct cpuinfo_x86 *c)
> >  {
> >  	set_cpu_cap(c, X86_FEATURE_ZEN);
> >  
> 
> I'm thinking this deserves a comment. Traditionally the SLIT table held
> relative memory latency. So where the identity is 10, 16 would indicate
> 1.6 times local latency and 32 would be 3.2 times local.
> 
> Now, even very early on BIOS monkeys went about their business and put
> in random values in an attempt to 'tune' the system based on how
> $random-os behaved, which is all sorts of fu^Wwrong.
> 
> Now, I suppose my question is; is that 32 Zen puts in an actual relative
> memory latency metric, or a random value we somehow have to deal with.
> And can we pretty please describe the whole sordid story behind this
> 'tunable' somewhere?

This is one for the AMD folks. I don't know if the memory latency
really is 3.2 times or not, only that that's the value in all the Zen
machines I have access to. Even this 2-socket one:

node distances:
node   0   1 
  0:  10  32 
  1:  32  10 

Tom, Suravee?