linux-kernel - Re: [PATCH 03/11] x86 topology: Add CPUID.1F multi-die/package support

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Date:   Tue, 26 Feb 2019 14:54:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Len Brown <lenb@...nel.org>
Cc:     X86 ML <x86@...nel.org>, linux-kernel@...r.kernel.org,
        Len Brown <len.brown@...el.com>, linux-doc@...r.kernel.org
Subject: Re: [PATCH 03/11] x86 topology: Add CPUID.1F multi-die/package
 support

On Wed, Feb 20, 2019 at 10:08:48AM -0500, Len Brown wrote:
> Thanks for the comments, Peter. I'll update the patch to address the
> syntax points.  (Maybe checkpatch.pl should be updated to reflect your
> preferences?).

Don't know about checkpatch; I ignore plenty of its output. I think tglx
started a document somewhere for what tip prefers, but I'm not sure
where that went.

> About macros vs C.  I agree with your preference.
> I used macros to be consistent with the existing code, and to be as
> backport friendly as possible.
> (a number of distros need to pull these patches into their supported kernels)
> Sure, I'm willing to write in a cosmetic-only patch, after the
> functional changes are upstream.

Fair enough.

> > It would've been nice to have the CPUID instruction 1F leaf reference
> > 3B-3.9 in the SDM, and maybe mention this here too.
> 
> I didn't mention SDM sections because they change -- leaving stale
> pointers in our commit messages.  The SDM is re-published 4 times per
> year.

Yah, I know. Which is why I keep all SDMs. So if you say, book 3 section
8 of Jul'17, I can find it :-)

> > You haven't explained, and I can't readily find it in the SDM either,
> > how these topology bits relate to caches and interconnects.
> >
> > Will these die thingies share LLC, or will LLC be per die. Will NUMA
> > span dies or not.
> 
> Excellent question.
> Cache enumeration in Leaf-4 is totally unchanged.
> ACPI NUMA tables are totally unchanged.

Sure; and yet Sub-NUMA-Clustering broke stuff in interesting ways. I'm
trying to get a feel for how these levels will interact with all that.

Before that SNC stuff, caches had never spanned NODEs (and I still
think that is 'creative' at best).

> From a scheduler point of view, imagine that a SKX system with 4 die
> in 4 packages was mechanically re-designed so that those 4 die resided
> in 2 double-sized packages.
> 
> They may have tweaked the links between the die, but logically it is
> identical and compatible, and the legacy kernel will function
> properly.

This example has LLC in die and yes that works.

But I can imagine things like L2 in tile and L3 across tiles but within
DIE and then it _might_ make sense to still consider the tile for
scheduling.

Another option is having the LLC off die; also not unheard of.

And then there's many creative and slightly crazy ways this can all be
combined :/

> So the effect of Leaf B,1F is that it defines the scope of MSRs.  eg.
> what processors does a die-scope MSR cover.  That is why the rest of
> the patch is about sysfs topology, and about package MSR scope.
> 
> Yes, there will be more exotic MSR situations in future products --
> the first ones are pretty simple -- something  called a
> package-scope-MSR  in the SDM today becomes a die-scope-MSR in this
> generation on a multi-die/package system.

Yes :-(

> It also reflects how many packages appear in sysfs, and this can
> effect licensing of some kinds of software.

That's just plain insanity and we should not let that affect our sysfs
interfaces.