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 <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
