linux-kernel - Re: [PATCH] x86/platform/uv: Use alternate source for socket to node data

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Message-ID: <29cd0ee1-c638-9d8e-8a1c-8c2aa2e167e7@intel.com>
Date:   Thu, 7 Sep 2023 13:57:16 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Steve Wahl <steve.wahl@....com>
Cc:     Dimitri Sivanich <dimitri.sivanich@....com>,
        Russ Anderson <russ.anderson@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/platform/uv: Use alternate source for socket to node
 data

On 9/5/23 08:01, Steve Wahl wrote:
> On Fri, Sep 01, 2023 at 02:01:52PM -0500, Steve Wahl wrote:
>> On Fri, Sep 01, 2023 at 10:18:20AM -0700, Dave Hansen wrote:
>>> On 8/7/23 07:17, Steve Wahl wrote:
>>>> When nr_cpus is set to a smaller number than actually present, the
>>>> cpu_to_node() mapping information for unused CPUs is not available to
>>>> build_socket_tables().  This results in an incomplete table and will
>>>> later cause use of a -1 value for some array indexing, and eventual
>>>> kernel page faults.
>>>>
>>>> Switch to using the __apicid_to_node array, which still contains all
>>>> the information mapping apicids to nodes, even for CPUs disabled with
>>>> a reduced nr_cpus setting.
>>> Before, the lookup went:
>>>
>>> 	CPU => APICID => SOCKET
>>>
>>> But when the CPU wasn't present, there wasn't a way to start this lookup.
>>>
>>> So, instead of looping over all CPUs, looking up their APICIDs and
>>> mapping those to sockets, just take CPUs out of the equation entirely.
>>>
>>> Loop over all APICIDs which are mapped to a valid NUMA node.  Then just
>>> extract the socket-id from the APICID.
>>>
>>> Right?
>>>
>>> That seems sane enough.  It's also way less code than the previous approach.
>> Yes.  That's it precisely.  And, yes, way less code.
> Are you willling to give a "Reviewed-by:"?

Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>

Does this need a stable@ tag when it gets applied?