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

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Date:   Thu, 7 Sep 2023 16:25:35 -0500
From:   Steve Wahl <steve.wahl@....com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Steve Wahl <steve.wahl@....com>,
        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 Thu, Sep 07, 2023 at 01:57:16PM -0700, Dave Hansen wrote:
> 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?

I hadn't thought about that.  I think it meets the requirements in
stable-kernel-rules.rst.  And it looks like it should apply without
conflicts or prerequisites.  So it probably should.  Is there a way to
add a cc:stable tag at this point?

Thank you, 

--> Steve Wahl

-- 
Steve Wahl, Hewlett Packard Enterprise