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

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Date:   Tue, 5 Sep 2023 10:01:44 -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 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:"?

--> Steve

-- 
Steve Wahl, Hewlett Packard Enterprise