[PATCH rfc] nvme-tcp: support simple polling

[hch@lst.de (Christoph Hellwig)]

On Wed, Jul 03, 2019@02:08:04PM -0700, Sagi Grimberg wrote:
> Simple polling support via socket busy_poll interface.
> Although we do not shutdown interrupts but simply hammer
> the socket poll, we can sometimes find completions faster
> than the normal interrupt driven RX path.
> 
> We add per queue nr_cqe counter that resets every time
> RX path is invoked such that .poll callback can return it
> to stay consistent with the semantics.
> 
> Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
> ---
> There is quite a bit more to do in this direction such as
> asking the net device to allocate polling rings and steer our
> polling queues to them, but that would require some infrastructure
> work in the networking stack.
> 
> With this alone, even while still absorbing the net device interrupts
> I'm seeing around 10%-20% latency improvement while sacrificing lots
> of cpu cycles for it.

Looks good enough to me, especially now that we need to explicitly opt
into polling queues.