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

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Date:   Mon, 7 Nov 2016 15:07:46 +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 08:31:21AM +0100, Ingo Molnar wrote:
> > > cpu_llc_id (Last Level Cache ID) derivation on AMD Fam17h has an
> > > underflow bug when extracting the socket_id value. It starts from 0
> 
> How's this...
> 
> > > so subtracting 1 from it will result in an invalid value. This breaks
> > > scheduling topology later on since the cpu_llc_id will be incorrect.
>    ^^^^^^^^^^^^^^^^^^^
> ... here?
> 
> > Same question as for the previous patch: what are the effects of the bug:
> 
> See above.

There's many ways the scheduling topology can 'break', resulting in different 
effects:

 - scheduling domains might be mixed up to the extent of crashing the bootup

 - some cores might be missing altogether, reducing available CPUs in essence

 - 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)

... do we know which of these occurs in this situation?

Thanks,

	Ingo