linux-kernel - Re: [PATCH 7/7] perf/x86/intel/P4: Fix smp_num

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Date:   Tue, 16 Aug 2022 10:26:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhang Rui <rui.zhang@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-hwmon@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        corbet@....net, fenghua.yu@...el.com, jdelvare@...e.com,
        linux@...ck-us.net, len.brown@...el.com
Subject: Re: [PATCH 7/7] perf/x86/intel/P4: Fix smp_num_siblings usage

On Tue, Aug 16, 2022 at 10:26:19AM +0800, Zhang Rui wrote:
> On Mon, 2022-08-15 at 11:11 +0200, Peter Zijlstra wrote:
> > On Sat, Aug 13, 2022 at 12:41:44AM +0800, Zhang Rui wrote:
> > > smp_num_siblings can be larger than 2.
> > 
> > Not on a P4 it can't ;-)
> 
> Kernel code doesn't prevent this from happening, so it just depends on
> how SMT ID is encoded in APICID.

No P4 ever encountered had it different and since no P4 will ever be
made again (I sincerely hope) we have a complete case.

> Checking for smp_num_sibling > 1 is the right logic to detect HT
> support, which is followed by all other kernel code except this one. :)

I'm fine with making it consistent, I'm arguing with the subject calling
this a fix, it is not, it's a functional no-op.