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

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Date:   Mon, 19 Oct 2020 15:12:33 +0200
From:   Brice Goglin <Brice.Goglin@...ia.fr>
To:     Peter Zijlstra <peterz@...radead.org>,
        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,
        valentin.schneider@....com
Subject: Re: [RFC PATCH] topology: Represent clusters of CPUs within a die.


Le 19/10/2020 à 14:50, Peter Zijlstra a écrit :
> 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.


Only the sysfs filenames are deprecated:

thread_siblings -> core_cpus

core_siblings -> package_cpus

New names reflect better what has been implemented/documented in the
past (core_siblings=package_cpus are processors with same physical
package id). And that's indeed different from the core-mask you are
talking about above with Core2-Quad (that one has never been exposed
anywhere in sysfs, except in the L3 cpumap).

Brice