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

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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




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