Re: [Qemu-devel] [PATCH v2 for-2.3] numa: pc: fix default VCPU to node mapping

["Andreas Färber" <afaerber@suse.de>]

Am 18.03.2015 um 17:38 schrieb Igor Mammedov:
> since commit
>    dd0247e0 pc: acpi: mark all possible CPUs as enabled in SRAT
> Linux kernel actually tries to use CPU to Node mapping from
> QEMU provided SRAT table instead of discarding it, and that
> in some cases breaks build_sched_domains() which expects
> sane mapping where cores/threads belonging to the same socket
> are on the same NUMA node.
> 
> With current default round-robin mapping of VCPUs to nodes
> guest ends-up with cores/threads belonging to the same socket
> being on different NUMA nodes.
> 
> For example with following CLI:
> qemu-kvm -m 4G -smp 5,sockets=2,cores=4,threads=1,maxcpus=8 \
>          -numa node,nodeid=0 -numa node,nodeid=1
> 2.6.32 based kernels will hang on boot due to incorrectly build
> sched_group-s list in update_sd_lb_stats()
> so comment in QEMU justifying dumb default mapping:
>  "
>   guest OSes must cope with this anyway, because there are BIOSes
>   out there in real machines which also use this scheme.
>  "
> isn't really valid.
> 
> Replacing default mapping with a manual, where VCPUs belonging to
> the same socket are on the same NUMA node, fixes issue for
> guests which can't handle nonsense topology i.e. changing CLI to:
>   -numa node,nodeid=0,cpus=0-3 -numa node,nodeid=1,cpus=4-7
> 
> So instead of simply scattering VCPUs around nodes, map
> the same socket VCPUs to the same NUMA node, which is what
> guest would expect from a sane hardware/BIOS.
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> v2:
>   - add machine callback cpu_index_to_socket_id() and use it
>     instead of stub approach
> ---
>  hw/i386/pc.c          |  9 +++++++++
>  include/hw/boards.h   |  5 +++++
>  include/sysemu/numa.h |  3 ++-
>  numa.c                | 18 +++++++++++++-----
>  vl.c                  |  2 +-
>  5 files changed, 30 insertions(+), 7 deletions(-)

Looks great to me now, the hook name with _socket_id is perfect,

Reviewed-by: Andreas Färber <afaerber@suse.de>

but can we do that in three steps please? "machine:" adding callback and
default implementation, "numa:" switching to use it and "pc:" overriding
the new callback - not only nicer subjects but easier to cherry-pick and
bisect then.

Regards,
Andreas

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