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Date:   Wed, 6 Jun 2018 16:07:45 +0100
From:   Morten Rasmussen <morten.rasmussen@arm.com>
To:     Jeremy Linton <jeremy.linton@arm.com>
Cc:     Sudeep.Holla@arm.com, Will.Deacon@arm.com, Catalin.Marinas@arm.com,
        Robin.Murphy@arm.com, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, geert@linux-m68k.org,
        linux-acpi@vger.kernel.org, ard.biesheuvel@linaro.org
Subject: Re: [PATCH] arm64: topology: Avoid checking numa mask for scheduler
 MC selection
Message-ID: <20180606150745.GC8461@e105550-lin.cambridge.arm.com>
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 <be5f7066-0b44-ff11-37e9-87512df2294f@arm.com>
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On Wed, Jun 06, 2018 at 09:53:39AM -0500, Jeremy Linton wrote:
> On 06/06/2018 09:44 AM, Morten Rasmussen wrote:
> >On Tue, Jun 05, 2018 at 02:08:37PM -0500, Jeremy Linton wrote:
> >>The numa mask subset check has problems if !CONFIG_NUMA, over hotplug
> >>operations or during early boot. Lets disable the NUMA siblings checks
> >>for the time being, as NUMA in socket machines have LLC's that will
> >>assure that the scheduler topology isn't "borken".
> >
> >Could we add an explanation why the numa node mask check is needed in
> >the first place. >
> >IIUC, we have the check in case the LLC is shared across numa nodes as
> >this would cause core_siblings > cpumask_of_node() which breaks the
> >scheduler topology.
> 
> Yes, that sounds like a good idea, my comments probably assume that the
> reader has been part of these conversations.
> 
> >
> >While sharing LLC across numa nodes seems quite unusual, I think it is
> >allowed by ACPI. Those systems might already be broken before, so might
> >not change anything. It is just worth noting why the check should be
> >added back later.
> 
> Right, there isn't anything in ACPI that dictates a system topology
> restriction like this. Given that other architectures have built machines
> with large directory caches that span numa nodes the check was a safety
> measure.

Agreed, it seems that another architecture has recently merged support
for that: 1340ccfa9a9a