Re: [Qemu-arm] [PATCH for-2.10 05/23] numa: move source of default CPUs to NUMA node mapping into boards

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Mar 28, 2017 at 12:53:10PM +0200, Igor Mammedov wrote:
> On Tue, 28 Mar 2017 15:19:20 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Wed, Mar 22, 2017 at 02:32:30PM +0100, Igor Mammedov wrote:
> > > Originally CPU threads were by default assigned in
> > > round-robin fashion. However it was causing issues in
> > > guest since CPU threads from the same socket/core could
> > > be placed on different NUMA nodes.
> > > Commit fb43b73b (pc: fix default VCPU to NUMA node mapping)
> > > fixed it by grouping threads within a socket on the same node
> > > introducing cpu_index_to_socket_id() callback and commit
> > > 20bb648d (spapr: Fix default NUMA node allocation for threads)
> > > reused callback to fix similar issues for SPAPR machine
> > > even though socket doesn't make much sense there.
> > > 
> > > As result QEMU ended up having 3 default distribution rules
> > > used by 3 targets /virt-arm, spapr, pc/.
> > > 
> > > In effort of moving NUMA mapping for CPUs into possible_cpus,
> > > generalize default mapping in numa.c by making boards decide
> > > on default mapping and let them explicitly tell generic
> > > numa code to which node a CPU thread belongs to by replacing
> > > cpu_index_to_socket_id() with @cpu_index_to_instance_props()
> > > which provides default node_id assigned by board to specified
> > > cpu_index.
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
[snip]
> > > +static CpuInstanceProperties
> > > +virt_cpu_index_to_props(MachineState *ms, unsigned cpu_index)
> > > +{
> > > +    MachineClass *mc = MACHINE_GET_CLASS(ms);
> > > +    const CPUArchIdList *possible_cpus = mc->possible_cpu_arch_ids(ms);
> > > +
> > > +    assert(cpu_index < possible_cpus->len);
> > > +    return possible_cpus->cpus[cpu_index].props;;
> > > +}
> > > +
> > 
> > It seems a bit weird to have a machine specific hook to pull the
> > property information when one way or another it's coming from the
> > possible_cpus table, which is already constructed by a machine
> > specific hook.  Could we add a range or list of cpu_index values to
> > each possible_cpus entry instead, and have a generic lookup of the
> > right entry based on that?

[snip]
> > > -static unsigned pc_cpu_index_to_socket_id(unsigned cpu_index)
> > > +static CpuInstanceProperties
> > > +pc_cpu_index_to_props(MachineState *ms, unsigned cpu_index)
> > >  {
> > > -    X86CPUTopoInfo topo;
> > > -    x86_topo_ids_from_idx(smp_cores, smp_threads, cpu_index,
> > > -                          &topo);
> > > -    return topo.pkg_id;
> > > +    MachineClass *mc = MACHINE_GET_CLASS(ms);
> > > +    const CPUArchIdList *possible_cpus = mc->possible_cpu_arch_ids(ms);
> > > +
> > > +    assert(cpu_index < possible_cpus->len);
> > > +    return possible_cpus->cpus[cpu_index].props;;
> > 
> > Since the pc and arm version of this are basically identical, I wonder
> > if that should actually be the default implementation.  If we need it
> > at all.
> ARM is still moving target and props are not really defined for it yet,
> so I'd like to keep it separate for now and when it stabilizes we can think
> about generalizing it.

Fair enough.

Any thoughts on my more general query above

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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