linux-kernel - Re: [PATCH] arm64: topology: Avoid checking numa mask for scheduler MC selection

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Date:   Wed, 6 Jun 2018 09:53:39 -0500
From:   Jeremy Linton <jeremy.linton@....com>
To:     Morten Rasmussen <morten.rasmussen@....com>
Cc:     Sudeep.Holla@....com, Will.Deacon@....com, Catalin.Marinas@....com,
        Robin.Murphy@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, geert@...ux-m68k.org,
        linux-acpi@...r.kernel.org, ard.biesheuvel@...aro.org
Subject: Re: [PATCH] arm64: topology: Avoid checking numa mask for scheduler
 MC selection

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.