Re: [PATCH v1 4/7] util: Add write-only "node-affinity" property for ThreadContext

[David Hildenbrand]

On 29.09.22 13:13, Markus Armbruster wrote:
> David Hildenbrand <david@redhat.com> writes:
>
> > Let's make it easier to pin threads created via a ThreadContext to
> > all current CPUs belonging to given NUMA nodes.
> >
> > As "node-affinity" is simply a shortcut for setting "cpu-affinity", that
> > property cannot be read and if the CPUs for a node change due do CPU
> > hotplug, the CPU affinity will not get updated.
>
> Color me confused.

I adjusted that to:

    util: Add write-only "node-affinity" property for ThreadContext
    
    Let's make it easier to pin threads created via a ThreadContext to
    all CPUs currently belonging to a given set of NUMA nodes -- which is the
    common case.
    
    "node-affinity" is simply a shortcut for setting "cpu-affinity" manually
    to the list of CPUs belonging to the set of nodes. This property can only
    be written.
    
    A simple QEMU example to set the CPU affinity to Node 1 on a system with
    two NUMA nodes, 24 CPUs each:
        qemu-system-x86_64 -S \
          -object thread-context,id=tc1,node-affinity=1
    
    And we can query the cpu-affinity via HMP/QMP:
        (qemu) qom-get tc1 cpu-affinity
        [
            1,
            3,
            5,
            7,
            9,
            11,
            13,
            15,
            17,
            19,
            21,
            23,
            25,
            27,
            29,
            31,
            33,
            35,
            37,
            39,
            41,
            43,
            45,
            47
        ]
    
    We cannot query the node-affinity:
        (qemu) qom-get tc1 node-affinity
        Error: Insufficient permission to perform this operation
    
    But note that due to dynamic library loading this example will not work
    before we actually make use of thread_context_create_thread() in QEMU
    code, because the type will otherwise not get registered.
    
    Note that if the CPUs for a node change due do physical CPU hotplug or
    hotunplug (e.g., lscpu output changes) after the ThreadContext was started,
    the CPU affinity will not get updated.



> > Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
> > Signed-off-by: David Hildenbrand <david@redhat.com>
> > ---
> >   qapi/qom.json         |  7 +++-
> >   util/meson.build      |  2 +-
> >   util/thread-context.c | 84 +++++++++++++++++++++++++++++++++++++++++++
> >   3 files changed, 91 insertions(+), 2 deletions(-)
> >
> > diff --git a/qapi/qom.json b/qapi/qom.json
> > index 4775a333ed..d36bf3355f 100644
> > --- a/qapi/qom.json
> > +++ b/qapi/qom.json
> > @@ -838,10 +838,15 @@
> >   # @cpu-affinity: the CPU affinity for all threads created in the thread
> >   #                context (default: QEMU main thread affinity)
> >   #
> > +# @node-affinity: shortcut for looking up the current CPUs for the given nodes
> > +#                 and setting @cpu-affinity (default: QEMU main thread
> > +#                 affinity)
> > +#
>
> Still confused.  Explain for dummies?  With an example, perhaps?

I adjusted that to:

+# @node-affinity: the list of node numbers that will be resolved to a list
+#                 of CPU numbers used as CPU affinity. This is a shortcut for
+#                 specifying the list of CPU numbers belonging to the nodes
+#                 manually by setting @cpu-affinity. (default: QEMU main thread
+#                 affinity)
+#

Thanks!

--
Thanks,

David / dhildenb