netdev - Re: [PATCH net-next RFC 5/5] vhost_net: basic tx virtqueue batched processing

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Date:   Thu, 28 Sep 2017 15:50:14 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH net-next RFC 5/5] vhost_net: basic tx virtqueue batched
 processing



On 2017年09月28日 08:55, Willem de Bruijn wrote:
>> @@ -461,6 +460,7 @@ static void handle_tx(struct vhost_net *net)
>>          struct socket *sock;
>>          struct vhost_net_ubuf_ref *uninitialized_var(ubufs);
>>          bool zcopy, zcopy_used;
>> +       int i, batched = VHOST_NET_BATCH;
>>
>>          mutex_lock(&vq->mutex);
>>          sock = vq->private_data;
>> @@ -475,6 +475,12 @@ static void handle_tx(struct vhost_net *net)
>>          hdr_size = nvq->vhost_hlen;
>>          zcopy = nvq->ubufs;
>>
>> +       /* Disable zerocopy batched fetching for simplicity */
> This special case can perhaps be avoided if we no longer block
> on vhost_exceeds_maxpend, but revert to copying.

Yes, I think so. For simplicity, I do it for data copy first. If the 
idea is convinced, I will try to do zerocopy on top.

>
>> +       if (zcopy) {
>> +               heads = &used;
> Can this special case of batchsize 1 not use vq->heads?

It doesn't in fact?

>
>> +               batched = 1;
>> +       }
>> +
>>          for (;;) {
>>                  /* Release DMAs done buffers first */
>>                  if (zcopy)
>> @@ -486,95 +492,114 @@ static void handle_tx(struct vhost_net *net)
>>                  if (unlikely(vhost_exceeds_maxpend(net)))
>>                          break;
>> +                       /* TODO: Check specific error and bomb out
>> +                        * unless ENOBUFS?
>> +                        */
>> +                       err = sock->ops->sendmsg(sock, &msg, len);
>> +                       if (unlikely(err < 0)) {
>> +                               if (zcopy_used) {
>> +                                       vhost_net_ubuf_put(ubufs);
>> +                                       nvq->upend_idx =
>> +                                  ((unsigned)nvq->upend_idx - 1) % UIO_MAXIOV;
>> +                               }
>> +                               vhost_discard_vq_desc(vq, 1);
>> +                               goto out;
>> +                       }
>> +                       if (err != len)
>> +                               pr_debug("Truncated TX packet: "
>> +                                       " len %d != %zd\n", err, len);
>> +                       if (!zcopy) {
>> +                               vhost_add_used_idx(vq, 1);
>> +                               vhost_signal(&net->dev, vq);
>> +                       } else if (!zcopy_used) {
>> +                               vhost_add_used_and_signal(&net->dev,
>> +                                                         vq, head, 0);
> While batching, perhaps can also move this producer index update
> out of the loop and using vhost_add_used_and_signal_n.

Yes.

>
>> +                       } else
>> +                               vhost_zerocopy_signal_used(net, vq);
>> +                       vhost_net_tx_packet(net);
>> +                       if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>> +                               vhost_poll_queue(&vq->poll);
>> +                               goto out;
>>                          }
>> -                       vhost_discard_vq_desc(vq, 1);
>> -                       break;
>> -               }
>> -               if (err != len)
>> -                       pr_debug("Truncated TX packet: "
>> -                                " len %d != %zd\n", err, len);
>> -               if (!zcopy_used)
>> -                       vhost_add_used_and_signal(&net->dev, vq, head, 0);
>> -               else
>> -                       vhost_zerocopy_signal_used(net, vq);
>> -               vhost_net_tx_packet(net);
>> -               if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
>> -                       vhost_poll_queue(&vq->poll);
>> -                       break;
> This patch touches many lines just for indentation. If having to touch
> these lines anyway (dirtying git blame), it may be a good time to move
> the processing of a single descriptor code into a separate helper function.
> And while breaking up, perhaps another helper for setting up ubuf_info.
> If you agree, preferably in a separate noop refactor patch that precedes
> the functional changes.

Right and it looks better, will try to do this.

Thanks