Re: [PATCH net-next] virtio-net: per-queue RPS config

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 17, 2019 at 10:37:26PM -0500, Willem de Bruijn wrote:
> On Thu, Jan 17, 2019 at 8:43 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Jan 17, 2019 at 08:08:53PM -0500, Willem de Bruijn wrote:
> > > From: Willem de Bruijn <willemb@google.com>
> > >
> > > On multiqueue network devices, RPS maps are configured independently
> > > for each receive queue through /sys/class/net/$DEV/queues/rx-*.
> > >
> > > On virtio-net currently all packets use the map from rx-0, because the
> > > real rx queue is not known at time of map lookup by get_rps_cpu.
> > >
> > > Call skb_record_rx_queue in the driver rx path to make lookup work.
> > >
> > > Recording the receive queue has ramifications beyond RPS, such as in
> > > sticky load balancing decisions for sockets (skb_tx_hash) and XPS.
> > >
> > > Reported-by: Mark Hlady <mhlady@google.com>
> > > Signed-off-by: Willem de Bruijn <willemb@google.com>
> >
> > And any examples how to see the benefit of this?
> 
> When there are fewer queues than cpus and rps is used to spread load
> across all cpus, it can be preferable to setup disjoint sets, such
> that each cpu handling an rxq interrupt spreads to an exclusive set of
> neighbors instead of having all interrupt handling cores contend on
> all other cores' softnet_data.
> 
> More subtly, even if the policy is to spread uniformly, it can be
> preferable to set the RPS map to all cores except the core that
> handled the interrupt, as it already had to do some work in the
> initial receive path.
> 
> It is also simply expected behavior for network devices to be able to
> configure rxq rps maps individually, so the current silent fallback to
> rx0 is confusing, especially since rx-1/rps_cpus, .. rx-n/rps_cpus
> files do exist and can be configured.

OK I think I got it.

Acked-by: Michael S. Tsirkin <mst@redhat.com>