Re: [PATCH net-next] vhost_net: conditionally enable tx polling

[Jason Wang]

On 2017å11æ01æ 00:36, Michael S. Tsirkin wrote:
> On Tue, Oct 31, 2017 at 06:27:20PM +0800, Jason Wang wrote:
> > We always poll tx for socket, this is sub optimal since:
> >
> > - we only want to be notified when sndbuf is available
> > - this will slightly increase the waitqueue traversing time and more
> >    important, vhost could not benefit from commit
> >    commit 9e641bdcfa4e
> >    ("net-tun: restructure tun_do_read for better sleep/wakeup efficiency")
> >    even if we've stopped rx polling during handle_rx() since tx poll
> >    were still left in the waitqueue.
> >
> > Pktgen from a remote host to VM over mlx4 shows 5.5% improvements on
> > rx PPS. (from 1.27Mpps to 1.34Mpps)
> >
> > Cc: Wei Xu <wexu@xxxxxxxxxx>
> > Cc: Matthew Rosato <mjrosato@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> Now that vhost_poll_stop happens on data path
> a lot, I'd say
>          if (poll->wqh)
> there should be unlikely().

It has been there since 8241a1e466cd ("vhost_net: stop polling socket 
during rx processing"). So it will be used for rx path too which 
unlikely() does not work as well as the case in tx.


> >   drivers/vhost/net.c | 11 ++++++++---
> >   1 file changed, 8 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 68677d9..286c3e4 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -471,6 +471,7 @@ static void handle_tx(struct vhost_net *net)
> >   		goto out;
> >   
> >   	vhost_disable_notify(&net->dev, vq);
> > +	vhost_net_disable_vq(net, vq);
> >   
> >   	hdr_size = nvq->vhost_hlen;
> >   	zcopy = nvq->ubufs;
> > @@ -556,6 +557,8 @@ static void handle_tx(struct vhost_net *net)
> >   					% UIO_MAXIOV;
> >   			}
> >   			vhost_discard_vq_desc(vq, 1);
> > +			if (err == -EAGAIN)
> > +				vhost_net_enable_vq(net, vq);
> >   			break;
> >   		}
> >   		if (err != len)
> I would probably just enable it unconditionally here. Why not?

I thought we only care about the case of tun_sock_write_space() and for 
the errors other than -EAGAIN, they have nothing to do with polling.

> > @@ -1145,9 +1148,11 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >   		r = vhost_vq_init_access(vq);
> >   		if (r)
> >   			goto err_used;
> > -		r = vhost_net_enable_vq(n, vq);
> > -		if (r)
> > -			goto err_used;
> > +		if (index == VHOST_NET_VQ_RX) {
> > +			r = vhost_net_enable_vq(n, vq);
> > +			if (r)
> > +				goto err_used;
> > +		}
> >   
> >   		oldubufs = nvq->ubufs;
> >   		nvq->ubufs = ubufs;
> This last chunk seems questionable. If queue has stuff in it
> when we connect the backend, we'll miss a wakeup.
> I suspect this can happen during migration.

Unless qemu pass a tap which s already had pending tx packets.

I can remove this chuck, but if guest does not transmit any packet, rx 
can't benefit from this.

Thanks

> > --
> > 2.7.4