Patchwork [V3,4/6] vhost_net: determine whether or not to use zerocopy at one time

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Submitter Jason Wang
Date Sept. 2, 2013, 8:40 a.m.
Message ID <1378111261-14826-5-git-send-email-jasowang@redhat.com>
Download mbox | patch
Permalink /patch/271749/
State Accepted
Delegated to: David Miller
Headers show

Comments

Jason Wang - Sept. 2, 2013, 8:40 a.m.
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 <jasowang@redhat.com>
---
 drivers/vhost/net.c |   47 ++++++++++++++++++++---------------------------
 1 files changed, 20 insertions(+), 27 deletions(-)
Michael S. Tsirkin - Sept. 4, 2013, 11:59 a.m.
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 <jasowang@redhat.com>
> ---
>  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 logic behind (nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx
here?



> +				   && 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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Jason Wang - Sept. 5, 2013, 2:54 a.m.
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 <jasowang@redhat.com>
>> ---
>>  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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Michael S. Tsirkin - Sept. 23, 2013, 7:16 a.m.
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 <jasowang@redhat.com>
> >> ---
> >>  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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> > the body of a message to majordomo@vger.kernel.org
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Jason Wang - Sept. 26, 2013, 4:30 a.m.
On 09/23/2013 03:16 PM, Michael S. Tsirkin wrote:
> 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 <jasowang@redhat.com>
>>>> > >> ---
>>>> > >>  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.

Ok, I will do some test to see the impact.
>>> > >
>>> > > 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?
>

It was used to forbid the zerocopy when heads ring are full, but since
we have the limitation now, it could be removed.
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Jason Wang - Sept. 29, 2013, 9:36 a.m.
On 09/26/2013 12:30 PM, Jason Wang wrote:
> On 09/23/2013 03:16 PM, Michael S. Tsirkin wrote:
>> > 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 <jasowang@redhat.com>
>>>>>>>> >>>> > >> ---
>>>>>>>> >>>> > >>  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.
> Ok, I will do some test to see the impact.

Since the our of order completion will happen when switching between
zero copy and non zero copy. I test this by using two sessions of
netperf in burst mode, one with 1 byte TCP_RR another with 512 bytes of
TCP_RR. There's no difference with the patch applied.

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Patch

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
+				   && 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;
 			}