netdev - Re: [PATCH net-next-2.6] vhost: Restart tx poll when socket send queue is full

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Date:	Thu, 18 Feb 2010 18:00:07 -0800
From:	Sridhar Samudrala <sri@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] vhost: Restart tx poll when socket send
 queue is full

On Fri, 2010-02-19 at 00:30 +0200, Michael S. Tsirkin wrote:
> On Thu, Feb 18, 2010 at 12:59:11PM -0800, Sridhar Samudrala wrote:
> > When running guest to remote host TCP stream test using vhost-net
> > via tap/macvtap, i am seeing network transmit hangs. This happens
> > when handle_tx() returns because of the socket send queue full 
> > condition.
> > This patch fixes this by restarting tx poll when hitting this
> > condition.
> 
> 
> Thanks! I would like to better understand what happens exactly.
> Some questions below:
> 
> > 
> > Signed-off-by: Sridhar Samudrala <sri@...ibm.com>
> > 
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index 91a324c..82d4bbe 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -113,12 +113,16 @@ static void handle_tx(struct vhost_net *net)
> >  	if (!sock)
> >  		return;
> >  
> > -	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > -	if (wmem >= sock->sk->sk_sndbuf)
> > -		return;
> > -
> 
> The disadvantage here is that a spurious wakeup
> when queue is still full becomes more expensive.
> 
> >  	use_mm(net->dev.mm);
> >  	mutex_lock(&vq->mutex);
> > +
> > +	wmem = atomic_read(&sock->sk->sk_wmem_alloc);
> > +	if (wmem >= sock->sk->sk_sndbuf) {
> > +		tx_poll_start(net, sock);
> 
> Hmm. We already do
>                        if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
>                                 tx_poll_start(net, sock);
>                                 set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
>                                 break;
>                         }
> why does not this code trigger here?

This check is done only when the ring is empty(head == vq->num).
But we are breaking out of the loop here.
                if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
                        vhost_poll_queue(&vq->poll);
                        break;
                }

I guess tx_poll_start() is missing here. The following patch fixes
the hang and may be a better fix.

Signed-off-by: Sridhar Samudrala <sri@...ibm.com>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 4c89283..fe9d296 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -172,6 +172,7 @@ static void handle_tx(struct vhost_net *net)
 		vhost_add_used_and_signal(&net->dev, vq, head, 0);
 		total_len += len;
 		if (unlikely(total_len >= VHOST_NET_WEIGHT)) {
+			tx_poll_start(net, sock);
 			vhost_poll_queue(&vq->poll);
 			break;
 		}

Thanks
Sridhar

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