Re: [PATCH net-next] vhost_net: conditionally enable tx polling
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2017-10-31 16:36:04
Also in:
kvm, lkml
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 <redacted>
Cc: Matthew Rosato <redacted>
Signed-off-by: Jason Wang <redacted>
---
Now that vhost_poll_stop happens on data path
a lot, I'd say
if (poll->wqh)
there should be unlikely().
quoted hunk ↗ jump to hunk
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?
quoted hunk ↗ jump to hunk
@@ -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.
-- 2.7.4