[net-next,v2,2/5] virtio-net: transmit napi

On Mon, Apr 24, 2017 at 1:14 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> On Mon, Apr 24, 2017 at 01:05:45PM -0400, Willem de Bruijn wrote:
>> On Mon, Apr 24, 2017 at 12:40 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>> > On Fri, Apr 21, 2017 at 10:50:12AM -0400, Willem de Bruijn wrote:
>> >> >>> Maybe I was wrong, but according to Michael's comment it looks like he
>> >> >>> want
>> >> >>> check affinity_hint_set just for speculative tx polling on rx napi
>> >> >>> instead
>> >> >>> of disabling it at all.
>> >> >>>
>> >> >>> And I'm not convinced this is really needed, driver only provide affinity
>> >> >>> hint instead of affinity, so it's not guaranteed that tx and rx interrupt
>> >> >>> are in the same vcpus.
>> >> >>
>> >> >> You're right. I made the restriction broader than the request, to really
>> >> >> err
>> >> >> on the side of caution for the initial merge of napi tx. And enabling
>> >> >> the optimization is always a win over keeping it off, even without irq
>> >> >> affinity.
>> >> >>
>> >> >> The cycle cost is significant without affinity regardless of whether the
>> >> >> optimization is used.
>> >> >
>> >> >
>> >> > Yes, I noticed this in the past too.
>> >> >
>> >> >> Though this is not limited to napi-tx, it is more
>> >> >> pronounced in that mode than without napi.
>> >> >>
>> >> >> 1x TCP_RR for affinity configuration {process, rx_irq, tx_irq}:
>> >> >>
>> >> >> upstream:
>> >> >>
>> >> >> 1,1,1: 28985 Mbps, 278 Gcyc
>> >> >> 1,0,2: 30067 Mbps, 402 Gcyc
>> >> >>
>> >> >> napi tx:
>> >> >>
>> >> >> 1,1,1: 34492 Mbps, 269 Gcyc
>> >> >> 1,0,2: 36527 Mbps, 537 Gcyc (!)
>> >> >> 1,0,1: 36269 Mbps, 394 Gcyc
>> >> >> 1,0,0: 34674 Mbps, 402 Gcyc
>> >> >>
>> >> >> This is a particularly strong example. It is also representative
>> >> >> of most RR tests. It is less pronounced in other streaming tests.
>> >> >> 10x TCP_RR, for instance:
>> >> >>
>> >> >> upstream:
>> >> >>
>> >> >> 1,1,1: 42267 Mbps, 301 Gcyc
>> >> >> 1,0,2: 40663 Mbps, 445 Gcyc
>> >> >>
>> >> >> napi tx:
>> >> >>
>> >> >> 1,1,1: 42420 Mbps, 303 Gcyc
>> >> >> 1,0,2:  42267 Mbps, 431 Gcyc
>> >> >>
>> >> >> These numbers were obtained with the virtqueue_enable_cb_delayed
>> >> >> optimization after xmit_skb, btw. It turns out that moving that before
>> >> >> increases 1x TCP_RR further to ~39 Gbps, at the cost of reducing
>> >> >> 100x TCP_RR a bit.
>> >> >
>> >> >
>> >> > I see, so I think we can leave the affinity hint optimization/check for
>> >> > future investigation:
>> >> >
>> >> > - to avoid endless optimization (e.g we may want to share a single
>> >> > vector/napi for tx/rx queue pairs in the future) for this series.
>> >> > - tx napi is disabled by default which means we can do optimization on top.
>> >>
>> >> Okay. I'll drop the vi->affinity_hint_set from the patch set for now.
>> >
>> > I kind of like it, let's be conservative. But I'd prefer a comment
>> > near it explaining why it's there.
>>
>> I don't feel strongly. Was minutes away from sending a v3 with this
>> code reverted, but I'll reinstate it and add a comment. Other planned
>> changes based on Jason's feedback to v2:
>>
>>   v2 -> v3:
>>     - convert __netif_tx_trylock to __netif_tx_lock on tx napi poll
>>           ensure that the handler always cleans, to avoid deadlock
>>     - unconditionally clean in start_xmit
>>           avoid adding an unnecessary "if (use_napi)" branch
>>     - remove virtqueue_disable_cb in patch 5/5
>>           a noop in the common event_idx based loop
>
> Makes sense, thanks!

Great. Sent that, thanks.

The actual diff to v2 is quite small:


@@ -1196,14 +1198,10 @@ static netdev_tx_t start_xmit(struct sk_buff
*skb, struct net_device *dev)
        bool use_napi = sq->napi.weight;

        /* Free up any pending old buffers before queueing new ones. */
-       if (use_napi) {
-               if (kick)
-                       virtqueue_enable_cb_delayed(sq->vq);
-               else
-                       virtqueue_disable_cb(sq->vq);
-       } else {
-               free_old_xmit_skbs(sq);
-       }
+       free_old_xmit_skbs(sq);
+
+       if (use_napi && kick)
+               virtqueue_enable_cb_delayed(sq->vq);

(gmail will munge the identation, sorry)