linux-kernel - Re: [PATCH v3 1/2] x86/AMD: Fix cpu_llc

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Date:   Tue, 8 Nov 2016 07:31:45 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...e.de>
Cc:     x86@...nel.org, Yazen Ghannam <Yazen.Ghannam@....com>,
        linux-kernel@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v3 1/2] x86/AMD: Fix cpu_llc_id for AMD Fam17h systems


* Borislav Petkov <bp@...e.de> wrote:

> On Mon, Nov 07, 2016 at 03:07:46PM +0100, Ingo Molnar wrote:
> >  - cache domains might be seriously mixed up, resulting in serious drop in
> >    performance.
> >
> >  - or domains might be partitioned 'wrong' but not catastrophically
> >   wrong, resulting in a minor performance drop (if at all)
> 
> Something between the two.
> 
> Here's some debugging output from set_cpu_sibling_map():
> 
> [    0.202033] smpboot: set_cpu_sibling_map: cpu: 0, has_smt: 0, has_mp: 1
> [    0.202043] smpboot: set_cpu_sibling_map: first loop, llc(this): 65528, o: 0, llc(o): 65528
> [    0.202058] smpboot: set_cpu_sibling_map: first loop, link mask smt
> 
> so we link it into the SMT mask even if has_smt is off.
> 
> [    0.202067] smpboot: set_cpu_sibling_map: first loop, link mask llc
> [    0.202077] smpboot: set_cpu_sibling_map: second loop, llc(this): 65528, o: 0, llc(o): 65528
> [    0.202091] smpboot: set_cpu_sibling_map: second loop, link mask die
> 
> I've attached the debug diff.
> 
> And since those llc(o), i.e. the cpu_llc_id of the *other* CPU in the
> loops in set_cpu_sibling_map() underflows, we're generating the funniest
> thread_siblings masks and then when I run 8 threads of nbench, they get
> spread around the LLC domains in a very strange pattern which doesn't
> give you the normal scheduling spread one would expect for performance.
>
> And this is just one workload - I can't imagine what else might be
> influenced by this funkiness.
> 
> Oh and other things like EDAC use cpu_llc_id so they will be b0rked too.

So the point I tried to make is that to people doing -stable backporting decisions 
this description you just gave is much more valuable than the previous changelog.

> So we absolutely need to fix that cpu_llc_id thing.

Absolutely!

Thanks,

	Ingo