Daniel Henrique Barboza <danielhb...@gmail.com> writes:
> On 11/5/21 10:51, Nicholas Piggin wrote:
>> A configuration that specifies multiple nodes without distance info
>> results in the non-local points in the FORM2 matrix having a distance of
>> 0. This causes Linux to complain "Invalid distance value range" because
>> a node distance is smaller than the local distance.
>>
>> Fix this by building a simple local / remote fallback for points where
>> distance information is missing.
>
> Thanks for looking this up. I checked the output of this same scenario with
> a FORM1 guest and 4 distance-less NUMA nodes. This is what I got:
>
> [root@localhost ~]# 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
> [root@localhost ~]#
>
>
> With this patch we're getting '20' instead of '160' because you're using
> NUMA_DISTANCE_DEFAULT, while FORM1 will default this case to the maximum
> NUMA distance the kernel allows for that affinity (160).
where is that enforced? Do we know why FORM1 picked 160?
>
> I do not have strong feelings about changing this behavior between FORM1 and
> FORM2. I tested the same scenario with a x86_64 guest and they also uses '20'
> in this case as well, so far as QEMU goes using NUMA_DISTANCE_DEFAULT is
> consistent.
>
for FORM2 I would suggest 20 is the right value and it is also
consistent with other architectures.
> Aneesh is already in CC, so I believe he'll let us know if there's something
> we're missing and we need to preserve the '160' distance in FORM2 for this
> case as well.
>
> For now:
>
>
>>
>> Signed-off-by: Nicholas Piggin <npig...@gmail.com>
>> ---
>
>
> Reviewed-by: Daniel Henrique Barboza <danielhb...@gmail.com>
>
>
>
>> hw/ppc/spapr_numa.c | 22 +++++++++++++++++-----
>> 1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/hw/ppc/spapr_numa.c b/hw/ppc/spapr_numa.c
>> index 5822938448..56ab2a5fb6 100644
>> --- a/hw/ppc/spapr_numa.c
>> +++ b/hw/ppc/spapr_numa.c
>> @@ -546,12 +546,24 @@ static void
>> spapr_numa_FORM2_write_rtas_tables(SpaprMachineState *spapr,
>> * NUMA nodes, but QEMU adds the default NUMA node without
>> * adding the numa_info to retrieve distance info from.
>> */
>> - if (src == dst) {
>> - distance_table[i++] = NUMA_DISTANCE_MIN;
>> - continue;
We always initialized the local distance to be NUMA_DISTANCE_MIN
irrespective of what is specified via Qemu command line before? If so
then the above change will break that?
>> + distance_table[i] = numa_info[src].distance[dst];
>> + if (distance_table[i] == 0) {
we know distance_table[i] is == 0 here and ..
>> + /*
>> + * In case QEMU adds a default NUMA single node when the
>> user
>> + * did not add any, or where the user did not supply
>> distances,
>> + * the value will be 0 here. Populate the table with a
>> fallback
>> + * simple local / remote distance.
>> + */
>> + if (src == dst) {
>> + distance_table[i] = NUMA_DISTANCE_MIN;
>> + } else {
>> + distance_table[i] = numa_info[src].distance[dst];
>> + if (distance_table[i] < NUMA_DISTANCE_MIN) {
considering we reached here after checking distance_table[i] == 0 do we
need to do the above two lines?
>> + distance_table[i] = NUMA_DISTANCE_DEFAULT;
>> + }
>> + }
>> }
>> -
>> - distance_table[i++] = numa_info[src].distance[dst];
>> + i++;
>> }
>> }
