thr3ads.net - Linux Virtualization - [PATCH V3 4/6] vhost_net: determine whether or not to use zerocopy at one time [Sep 2013]

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Michael S. Tsirkin

2013-Sep-23 07:16 UTC

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

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 at 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
> > --
> > To unsubscribe from this list: send the line "unsubscribe
kvm" in
> > the body of a message to majordomo at vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html

Jason Wang

2013-Sep-26 04:30 UTC

head link

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

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 at
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.

Jason Wang

2013-Sep-29 09:36 UTC

head link

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

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 at 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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