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

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Message-ID: <20200113132214.GD10914@e105550-lin.cambridge.arm.com>
Date:   Mon, 13 Jan 2020 13:22:14 +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 Sun, Jan 12, 2020 at 01:22:02PM +0000, Valentin Schneider wrote:
> On 09/01/2020 10:52, Morten Rasmussen wrote:
> >> 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.
> > 
> 
> Right, the point I was trying to make is that node boundaries can be pretty
> much anything, so nodes can span over LLCs, or LLCs can span over nodes,
> which is why we need checks such as the one in arch_topology() that lets us
> build up a usable domain hierarchy (which cares about LLCs, at least at some
> level).

Indeed. The topology masks can't always be used as is to define the
sched_domain hierarchy.