linux-kernel - Re: [RFC PATCH] topology: Represent clusters of CPUs within a die.

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Date:   Mon, 19 Oct 2020 14:50:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc:     linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Len Brown <len.brown@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sudeep Holla <sudeep.holla@....com>, guohanjun@...wei.com,
        Will Deacon <will@...nel.org>, linuxarm@...wei.com,
        Brice Goglin <Brice.Goglin@...ia.fr>,
        valentin.schneider@....com
Subject: Re: [RFC PATCH] topology: Represent clusters of CPUs within a die.

On Mon, Oct 19, 2020 at 01:32:26PM +0100, Jonathan Cameron wrote:
> On Mon, 19 Oct 2020 12:35:22 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:

> > I'm confused by all of this. The core level is exactly what you seem to
> > want.
> 
> It's the level above the core, whether in an multi-threaded core
> or a single threaded core.   This may correspond to the level
> at which caches are shared (typically L3).  Cores are already well
> represented via thread_siblings and similar.  Extra confusion is that
> the current core_siblings (deprecated) sysfs interface, actually reflects
> the package level and ignores anything in between core and
> package (such as die on x86)

That seems wrong. core-mask should be whatever cores share L3. So on a
Intel Core2-Quad (just to pick an example) you should have 4 CPU in a
package, but only 2 CPUs for the core-mask.

It just so happens that L3 and package were the same for a long while in
x86 land, although recent chips started breaking that trend.

And I know nothing about the core-mask being depricated; it's what the
scheduler uses. It's not going anywhere.

So if your 'cluster' is a group of single cores (possibly with SMT) that
do not share cache but have a faster cache connection and you want them
to behave as-if they were a multi-core group that did share cache, then
core-mask it is.