Re: [PATCH v2 1/1] spapr_numa.c: fix FORM1 distance-less nodes

["Cédric Le Goater" <clg@kaod.org>]

On 11/10/21 13:39, Daniel Henrique Barboza wrote:
> Commit 71e6fae3a99 fixed an issue with FORM2 affinity guests with NUMA
> nodes in which the distance info is absent in
> machine_state->numa_state->nodes. This happens when QEMU adds a default
> NUMA node and when the user adds NUMA nodes without specifying the
> distances.
> 
> During the discussions of the forementioned patch [1] it was found that
> FORM1 guests were behaving in a strange way in the same scenario, with
> the kernel seeing the distances between the nodes as '160', as we can
> see in this example with 4 NUMA nodes without distance information:
> 
> $ numactl -H
> available: 4 nodes (0-3)
> (...)
> node distances:
> node   0   1   2   3
>    0:  10  160  160  160
>    1:  160  10  160  160
>    2:  160  160  10  160
>    3:  160  160  160  10
> 
> Turns out that we have the same problem with FORM1 guests - we are
> calculating associativity domain using zeroed values. And as it also
> turns out, the solution from 71e6fae3a99 applies to FORM1 as well.
> 
> This patch creates a wrapper called 'get_numa_distance' that contains
> the logic used in FORM2 to define node distances when this information
> is absent. This helper is then used in all places where we need to read
> distance information from machine_state->numa_state->nodes. That way
> we'll guarantee that the NUMA node distance is always being curated
> before being used.
> 
> After this patch, the FORM1 guest mentioned above will have the
> following topology:
> 
> $ numactl -H
> available: 4 nodes (0-3)
> (...)
> node distances:
> node   0   1   2   3
>    0:  10  20  20  20
>    1:  20  10  20  20
>    2:  20  20  10  20
>    3:  20  20  20  10
> 
> This is compatible with what FORM2 guests and other archs do in this
> case.
> 
> [1] https://lists.gnu.org/archive/html/qemu-devel/2021-11/msg01960.html
> 
> Fixes: 690fbe4295d5 ("spapr_numa: consider user input when defining associativity")
> CC: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> CC: Nicholas Piggin <npiggin@gmail.com>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> ---

queued for 6.2.

Thanks for the resend.

C.