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SubjectRe: [PATCH] arm64: topology: Avoid checking numa mask for scheduler MC selection
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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.


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