On Wed, Nov 01, 2017 at 08:51:36PM +0800, Jason Wang wrote:
>
>
> On 2017年11月01日 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 <w...@redhat.com>
> > > Cc: Matthew Rosato <mjros...@linux.vnet.ibm.com>
> > > Signed-off-by: Jason Wang <jasow...@redhat.com>
> > > ---
> > 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.
Worth testing, does not have to block this patch.
>
> >
> >
> > > 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.
We could thinkably get ENOMEM I guess. If we miss a code things
get stuck - It's just easier not to add extra code IMHO.
> > > @@ -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
Not sure I understand the last sentence. vhost will stay
polling the socket - why is that a problem?
> >
> >
> > > --
> > > 2.7.4
>
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