Subject: Re: [PATCH V3 4/6] vhost_net: determine whether or not to use zerocopy at one time

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 23 Sep 2013 10:16:20 +0300

On Thu, Sep 05, 2013 at 10:54:44AM +0800, Jason Wang wrote:
> On 09/04/2013 07:59 PM, Michael S. Tsirkin wrote:
> > On Mon, Sep 02, 2013 at 04:40:59PM +0800, Jason Wang wrote:
> >> Currently, even if the packet length is smaller than VHOST_GOODCOPY_LEN, if
> >> upend_idx != done_idx we still set zcopy_used to true and rollback this choice
> >> later. This could be avoided by determining zerocopy once by checking all
> >> conditions at one time before.
> >>
> >> Signed-off-by: Jason Wang <[email-masked]>
> >> ---
> >>  drivers/vhost/net.c |   47 ++++++++++++++++++++---------------------------
> >>  1 files changed, 20 insertions(+), 27 deletions(-)
> >>
> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> >> index 8a6dd0d..3f89dea 100644
> >> --- a/drivers/vhost/net.c
> >> +++ b/drivers/vhost/net.c
> >> @@ -404,43 +404,36 @@ static void handle_tx(struct vhost_net *net)
> >>  			       iov_length(nvq->hdr, s), hdr_size);
> >>  			break;
> >>  		}
> >> -		zcopy_used = zcopy && (len >= VHOST_GOODCOPY_LEN ||
> >> -				       nvq->upend_idx != nvq->done_idx);
> >> +
> >> +		zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN
> >> +				   && (nvq->upend_idx + 1) % UIO_MAXIOV !=
> >> +				      nvq->done_idx
> > Thinking about this, this looks strange.
> > The original idea was that once we start doing zcopy, we keep
> > using the heads ring even for short packets until no zcopy is outstanding.
> 
> What's the reason for keep using the heads ring?

To keep completions in order.

> >
> > What's the logic behind (nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx
> > here?
> 
> Because we initialize both upend_idx and done_idx to zero, so upend_idx
> != done_idx could not be used to check whether or not the heads ring
> were full.

But what does ring full have to do with zerocopy use?

> >> +				   && vhost_net_tx_select_zcopy(net);
> >>  
> >>  		/* use msg_control to pass vhost zerocopy ubuf info to skb */
> >>  		if (zcopy_used) {
> >> +			struct ubuf_info *ubuf;
> >> +			ubuf = nvq->ubuf_info + nvq->upend_idx;
> >> +
> >>  			vq->heads[nvq->upend_idx].id = head;
> >> -			if (!vhost_net_tx_select_zcopy(net) ||
> >> -			    len < VHOST_GOODCOPY_LEN) {
> >> -				/* copy don't need to wait for DMA done */
> >> -				vq->heads[nvq->upend_idx].len =
> >> -							VHOST_DMA_DONE_LEN;
> >> -				msg.msg_control = NULL;
> >> -				msg.msg_controllen = 0;
> >> -				ubufs = NULL;
> >> -			} else {
> >> -				struct ubuf_info *ubuf;
> >> -				ubuf = nvq->ubuf_info + nvq->upend_idx;
> >> -
> >> -				vq->heads[nvq->upend_idx].len =
> >> -					VHOST_DMA_IN_PROGRESS;
> >> -				ubuf->callback = vhost_zerocopy_callback;
> >> -				ubuf->ctx = nvq->ubufs;
> >> -				ubuf->desc = nvq->upend_idx;
> >> -				msg.msg_control = ubuf;
> >> -				msg.msg_controllen = sizeof(ubuf);
> >> -				ubufs = nvq->ubufs;
> >> -				kref_get(&ubufs->kref);
> >> -			}
> >> +			vq->heads[nvq->upend_idx].len = VHOST_DMA_IN_PROGRESS;
> >> +			ubuf->callback = vhost_zerocopy_callback;
> >> +			ubuf->ctx = nvq->ubufs;
> >> +			ubuf->desc = nvq->upend_idx;
> >> +			msg.msg_control = ubuf;
> >> +			msg.msg_controllen = sizeof(ubuf);
> >> +			ubufs = nvq->ubufs;
> >> +			kref_get(&ubufs->kref);
> >>  			nvq->upend_idx = (nvq->upend_idx + 1) % UIO_MAXIOV;
> >> -		} else
> >> +		} else {
> >>  			msg.msg_control = NULL;
> >> +			ubufs = NULL;
> >> +		}
> >>  		/* TODO: Check specific error and bomb out unless ENOBUFS? */
> >>  		err = sock->ops->sendmsg(NULL, sock, &msg, len);
> >>  		if (unlikely(err < 0)) {
> >>  			if (zcopy_used) {
> >> -				if (ubufs)
> >> -					vhost_net_ubuf_put(ubufs);
> >> +				vhost_net_ubuf_put(ubufs);
> >>  				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
> >>  					% UIO_MAXIOV;
> >>  			}
> >> -- 
> >> 1.7.1
> > --
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