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SubjectRe: [PATCH] x86, sched: Allow NUMA nodes to share an LLC on Intel platforms
On Tue, Feb 09, 2021 at 11:09:27PM +0000, Luck, Tony wrote:
> > +#define X86_BUG_NUMA_SHARES_LLC X86_BUG(25) /* CPU may enumerate an LLC shared by multiple NUMA nodes */
>
> During internal review I wondered why this is a "BUG" rather than a "FEATURE" bit.
>
> Apparently, the suggestion for "BUG" came from earlier community discussions.
>
> Historically it may have seemed reasonable to say that a cache cannot span
> NUMA domains. But with more and more things moving off the motherboard
> and into the socket, this doesn't seem too weird now.

If you look at the details this SNC LLC span doesn't behave quite right
either.

It really isn't a regular cache, but behaves a bit like a mash-up of the
s390 book caches and a normal LLC.

Did anybody play with adding the book domain to these SNC
configurations? Can we detect SNC other than by this quirk?

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