Re: [PATCH 1/2] numa: Set default distance map if needed

[Andrew Jones]

On Fri, Oct 08, 2021 at 10:51:24AM +1100, Gavin Shan wrote:
> Hi Drew,
> 
> On 10/6/21 10:56 PM, Andrew Jones wrote:
> > On Wed, Oct 06, 2021 at 10:03:25PM +1100, Gavin Shan wrote:
> > > On 10/6/21 9:35 PM, Andrew Jones wrote:
> > > > On Wed, Oct 06, 2021 at 06:22:08PM +0800, Gavin Shan wrote:
> > > > > The following option is used to specify the distance map. It's
> > > > > possible the option isn't provided by user. In this case, the
> > > > > distance map isn't populated and exposed to platform. On the
> > > > > other hand, the empty NUMA node, where no memory resides, is
> > > > > allowed on ARM64 virt platform. For these empty NUMA nodes,
> > > > > their corresponding device-tree nodes aren't populated, but
> > > > > their NUMA IDs should be included in the "/distance-map"
> > > > > device-tree node, so that kernel can probe them properly if
> > > > > device-tree is used.
> > > > > 
> > > > >     -numa,dist,src=<numa_id>,dst=<numa_id>,val=<distance>
> > > > > 
> > > > > So when user doesn't specify distance map, we need to generate
> > > > > the default distance map, where the local and remote distances
> > > > > are 10 and 20 separately. This adds an extra parameter to the
> > > > > exiting complete_init_numa_distance() to generate the default
> > > > > distance map for this case.
> > > > > 
> > > > > Signed-off-by: Gavin Shan <gshan@redhat.com>
> > > > > ---
> > > > >    hw/core/numa.c | 13 +++++++++++--
> > > > >    1 file changed, 11 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/hw/core/numa.c b/hw/core/numa.c
> > > > > index 510d096a88..fdb3a4aeca 100644
> > > > > --- a/hw/core/numa.c
> > > > > +++ b/hw/core/numa.c
> > > > > @@ -594,7 +594,7 @@ static void validate_numa_distance(MachineState 
> > > > > *ms)
> > > > >        }
> > > > >    }
> > > > > -static void complete_init_numa_distance(MachineState *ms)
> > > > > +static void complete_init_numa_distance(MachineState *ms, bool 
> > > > > is_default)
> > > > >    {
> > > > >        int src, dst;
> > > > >        NodeInfo *numa_info = ms->numa_state->nodes;
> > > > > @@ -609,6 +609,8 @@ static void 
> > > > > complete_init_numa_distance(MachineState *ms)
> > > > >                if (numa_info[src].distance[dst] == 0) {
> > > > >                    if (src == dst) {
> > > > >                        numa_info[src].distance[dst] = 
> > > > > NUMA_DISTANCE_MIN;
> > > > > +                } else if (is_default) {
> > > > > +                    numa_info[src].distance[dst] = 
> > > > > NUMA_DISTANCE_DEFAULT;
> > > > >                    } else {
> > > > >                        numa_info[src].distance[dst] = 
> > > > > numa_info[dst].distance[src];
> > > > >                    }
> > > > > @@ -716,13 +718,20 @@ void numa_complete_configuration(MachineState 
> > > > > *ms)
> > > > >             * A->B != distance B->A, then that means the distance 
> > > > > table is
> > > > >             * asymmetric. In this case, the distances for both 
> > > > > directions
> > > > >             * of all node pairs are required.
> > > > > +         *
> > > > > +         * The default node pair distances, which are 10 and 20 for 
> > > > > the
> > > > > +         * local and remote nodes separatly, are provided if user 
> > > > > doesn't
> > > > > +         * specify any node pair distances.
> > > > >             */
> > > > >            if (ms->numa_state->have_numa_distance) {
> > > > >                /* Validate enough NUMA distance information was 
> > > > > provided. */
> > > > >                validate_numa_distance(ms);
> > > > >                /* Validation succeeded, now fill in any missing 
> > > > > distances. */
> > > > > -            complete_init_numa_distance(ms);
> > > > > +            complete_init_numa_distance(ms, false);
> > > > > +        } else {
> > > > > +            complete_init_numa_distance(ms, true);
> > > > > +            ms->numa_state->have_numa_distance = true;
> > > > >            }
> > > > >        }
> > > > >    }
> > > > > -- 
> > > > > 2.23.0
> > > > > 
> > > > 
> > > > With this patch we'll always generate a distance map when there's a numa
> > > > config now. Is there any reason a user would not want to do that? I.e.
> > > > should we still give the user the choice of presenting a distance map?
> > > > Also, does the addition of a distance map in DTs for compat machine 
> > > > types
> > > > matter?
> > > > 
> > > > Otherwise patch looks good to me.
> > > > 
> > > 
> > > Users needn't specify the distance map when the default one in kernel,
> > > whose distances are 10 and 20 for local and remote nodes in linux for
> > > all architectures and machines, is used. The following option is still
> > > usable to specify the distance map.
> > > 
> > >    -numa,dist,src=<numa_id>,dst=<numa_id>,val=<distance>
> > > 
> > > When the empty NUMA nodes are concerned, the distance map is mandatory
> > > because their NUMA IDs are identified from there. So we always generate
> > > the distance map as this patch does :)
> > > 
> > 
> > Yup, I knew all that already :-) I'm asking if we want to ensure the user
> > can still control whether or not this distance map is generated at all. If
> > a user doesn't want empty numa nodes or a distance map, then, with this
> > patch, they cannot avoid the map's generation. That configurability
> > question also relates to machine compatibility. Do we want to start
> > generating this distance map on old, numa configured machine types? This
> > patch will do that too.
> > 
> > But, it might be OK to just start generating this new DT node for all numa
> > configured machine types and not allow the user to opt out. I do know that
> > we allow hardware descriptions to be changed without compat code.  Also, a
> > disable-auto-distance-map option may be considered useless and therefore
> > not worth maintaining. The conservative in me says it's worth debating
> > these things first though.
> > 
> > (Note, empty numa nodes have never worked with QEMU, so it's OK to start
> >   erroring out when empty numa nodes and a disable-auto-distance-map option
> >   are given together.)
> > 
> 
> Sorry for the delay. I didn't fully understand "machine compatibility" even
> after checking the code around. Could you please provide more details? I'm
> not sure if the enforced distance-map for empty NUMA nodes will cause any
> issues?

On QEMU, currently booting/running VMs on machine type X should not notice
when QEMU has been updated and they are still boot with machine type X.
That's what the "compat machine types" stuff means and what I'm referring
to above. I think it may be fine to boot a VM that never had a
distance-map before on an updated QEMU with machine type X and suddenly
get a distance-map, because we claim this is similar to a firmware update
that will change hardware descriptions on reboot. We expect guest kernels
to be tolerant of that. That said, there's always some risk, so we need
to consciously make that decision. Also, if we choose to expose a switch
to disable to the auto-distance-map to the user, then it's pretty trivial
to automatically set that on older machine types in order to avoid the
concern. So, do we think we need to expose a disable-auto-distance-map
type of option? Or would that be a useless burden? Also, if the decision
is to not worry about it, then the commit message should be updated to
add the rationale for that decision.

> 
> Yes, the empty NUMA node never worked with QEMU if device-tree is used.
> We still need to figure out a way to support memory hotplug through
> device-tree, similar thing as to what IBM's pSeries platform has.

That's for the guest kernel to figure out. I doubt it'll be a high
priority, though, because, as you've shown below, memory hotplug works
with ACPI, which is what Arm servers use. I don't expect smaller DT
platforms to care much about memory hotplug.

Thanks,
drew

> However, it works when ACPI table is used. Taking the following
> command line as an example, the hot-added memory is always put
> into the last NUMA node (3). The last NUMA node can be empty node
> after changing the code to allow to export ACPI SRAT table to include
> the empty NUMA nodes.
> 
>    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
>    -accel kvm -machine virt,gic-version=host               \
>    -cpu host -smp 4,sockets=2,cores=2,threads=1            \
>    -m 1024M,slots=16,maxmem=64G                            \
>    -object memory-backend-ram,id=mem0,size=512M            \
>    -object memory-backend-ram,id=mem1,size=512M            \
>    -numa node,nodeid=0,cpus=0-1,memdev=mem0                \
>    -numa node,nodeid=1,cpus=2-3,memdev=mem1                \
>    -numa node,nodeid=2                                     \
>    -numa node,nodeid=3
>      :
>      :
>    guest# cat /sys/devices/system/node/node3/meminfo | grep MemTotal
>    Node 3 MemTotal:              0 kB
>    (qemu) object_add memory-backend-ram,id=hpmem0,size=1G
>    (qemu) device_add pc-dimm,id=dimm1,memdev=hpmem0,node=3
>    guest# cat /sys/devices/system/node/node3/meminfo | grep MemTotal
>    Node 3 MemTotal:        1048576 kB
> 
> Thanks,
> Gavin
> 
> 
>