Re: [PATCH net-next] virtio-net: enable multiqueue by default

[Jason Wang]

On 2016å11æ25æ 12:43, Michael S. Tsirkin wrote:
> On Fri, Nov 25, 2016 at 12:37:26PM +0800, Jason Wang wrote:
> > >We use single queue even if multiqueue is enabled and let admin to
> > >enable it through ethtool later. This is used to avoid possible
> > >regression (small packet TCP stream transmission). But looks like an
> > >overkill since:
> > >
> > >- single queue user can disable multiqueue when launching qemu
> > >- brings extra troubles for the management since it needs extra admin
> > >   tool in guest to enable multiqueue
> > >- multiqueue performs much better than single queue in most of the
> > >   cases
> > >
> > >So this patch enables multiqueue by default: if #queues is less than or
> > >equal to #vcpu, enable as much as queue pairs; if #queues is greater
> > >than #vcpu, enable #vcpu queue pairs.
> > >
> > >Cc: Hannes Frederic Sowa<hannes@xxxxxxxxxx>
> > >Cc: Michael S. Tsirkin<mst@xxxxxxxxxx>
> > >Cc: Neil Horman<nhorman@xxxxxxxxxx>
> > >Cc: Jeremy Eder<jeder@xxxxxxxxxx>
> > >Cc: Marko Myllynen<myllynen@xxxxxxxxxx>
> > >Cc: Maxime Coquelin<maxime.coquelin@xxxxxxxxxx>
> > >Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> OK at some level but all uses of num_online_cpus()
> like this are racy versus hotplug.
> I know we already have this bug but shouldn't we fix it
> before we add more?

Not sure I get the point, do you mean adding get/put_online_cpus()? But 
is it a real bug? We don't do any cpu specific things so I believe it's 
not necessary (unless we want to keep #queues == #vcpus magically but I 
don't think so). Admin need to re-configure #queues after cpu hotplug if 
they wish.

Thanks