linux-kernel - Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts with lower layer

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Message-ID: <20200109105228.GB10914@e105550-lin.cambridge.arm.com>
Date:   Thu, 9 Jan 2020 10:52:28 +0000
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Sudeep Holla <sudeep.holla@....com>,
        Linuxarm <linuxarm@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpu-topology: warn if NUMA configurations conflicts with
 lower layer

On Thu, Jan 02, 2020 at 01:22:19PM +0000, Valentin Schneider wrote:
> On 02/01/2020 12:47, Zengtao (B) wrote:
> >>
> >> As I said, wrong configurations need to be detected when generating
> >> DT/ACPI if possible. The above will print warning on systems with NUMA
> >> within package.
> >>
> >> NUMA:  0-7, 8-15
> >> core_siblings:   0-15
> >>
> >> The above is the example where the die has 16 CPUs and 2 NUMA nodes
> >> within a package, your change throws error to the above config which is
> >> wrong.
> >>
> > From your example, the core 7 and core 8 has got different LLC but the same Low
> > Level cache?
> 
> AFAIA what matters here is memory controllers, less so LLCs. Cores within
> a single die could have private LLCs and separate memory controllers, or
> shared LLC and separate memory controllers.

Don't confuse cache boundaries, packages and nodes :-)

core_siblings are cpus in the same package and doesn't say anything
about cache boundaries. It is not given that there is sched_domain that
matches the core_sibling span.

The MC sched_domain is supposed to match the LLC span which might
different for core_siblings. So the about example should be valid for a
NUMA-in-package system with one package containing two nodes.

Morten