linux-kernel - RE: Regression introduced by cf6d445f68974d0b15a14cf6021be38a91f2b5d8

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Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077058FAD81@SHSMSX103.ccr.corp.intel.com>
Date:	Mon, 9 May 2016 20:55:22 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Thomas Gleixner <tglx@...utronix.de>, Josef Bacik <jbacik@...com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	kernel-team <Kernel-team@...com>,
	"x86@...nel.org" <x86@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: RE: Regression introduced by
 cf6d445f68974d0b15a14cf6021be38a91f2b5d8


> Josef,
> 
> On Mon, 9 May 2016, Josef Bacik wrote:
> 
> > I've hit a regression that was introduced by the commit in $SUBJECT.
> > I'm building a minimal kernel config that doesn't have CONFIG_SMP set,
> > which results in the topology for the box to look different than with
> > CONFIG_SMP set. Specifically boot_cpu_data.x86_max_cores comes out to
> > 16 without CONFIG_SMP set, but 12 with CONFIG_SMP set.
> 
> So the commit is just exposing the underlying wreckage.
> 
> x86_max_cores is set via:
> 
>       detect_extended_topology(), which is a NOOP in case of SMP=n
> 
> or
> 
>       if the above is not available, which is the case for SMP=n, then it
>       uses:
> 
>       intel_num_cpu_cores()
> 
>       That's using the cache leaf Bits 26-31:
> 
>       Maximum number of addressable IDs for processor cores in the physical
>       package
> 
>       And that's 16 for this CPU, but that has nothing to do with the actual
>       number of cores in the package.
> 
> So that explains the wreckage you are seing. We have two options to deal
> with
> this:
> 
> 1) Make intel_num_cpu_cores() a NOOP for SMP=n, so x86_max_cores = 1
> 
> 2) Make detect_extended_topology() functional for SMP=n, so the real
> number of
>    cores is detected
> 
> Both options work and make sense.
> 
> Though I don't know whether the uncore stuff wants to see all boxes of a
> package even in the SMP=n case. Kan?

I think the boxes num should not be larger than the max cores.
So if SMP=n, there is only one core. The boxes num should be one as well.
Option one looks good to me.

Thanks,
Kan