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

From: Jason Wang <[email-masked]>
Date: Thu, 05 Sep 2013 10:54:44 +0800

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?
>
> 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.
>> +				   && 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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