Subject: Re: [PATCH for-2.6.35] virtio_net: fix oom handling on tx

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 10 Jun 2010 22:03:43 +0300

On Thu, Jun 10, 2010 at 10:46:53AM -0700, Stephen Hemminger wrote:
> On Thu, 10 Jun 2010 10:17:07 -0700
> Sridhar Samudrala <[email-masked]> wrote:
> 
> > On Thu, 2010-06-10 at 18:20 +0300, Michael S. Tsirkin wrote:
> > > virtio net will never try to overflow the TX ring, so the only reason
> > > add_buf may fail is out of memory. Thus, we can not stop the
> > > device until some request completes - there's no guarantee anything
> > > at all is outstanding.
> > > 
> > > Make the error message clearer as well: error here does not
> > > indicate queue full.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > > ---
> > >  drivers/net/virtio_net.c |   15 ++++++++-------
> > >  1 files changed, 8 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 85615a3..e48a06f 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -563,7 +563,6 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> > >  	struct virtnet_info *vi = netdev_priv(dev);
> > >  	int capacity;
> > > 
> > > -again:
> > >  	/* Free up any pending old buffers before queueing new ones. */
> > >  	free_old_xmit_skbs(vi);
> > > 
> > > @@ -572,12 +571,14 @@ again:
> > > 
> > >  	/* This can happen with OOM and indirect buffers. */
> > >  	if (unlikely(capacity < 0)) {
> > > -		netif_stop_queue(dev);
> > > -		dev_warn(&dev->dev, "Unexpected full queue\n");
> > > -		if (unlikely(!virtqueue_enable_cb(vi->svq))) {
> > > -			virtqueue_disable_cb(vi->svq);
> > > -			netif_start_queue(dev);
> > > -			goto again;
> > > +		if (net_ratelimit()) {
> > > +			if (likely(capacity == -ENOMEM))
> > > +				dev_warn(&dev->dev,
> > > +					 "TX queue failure: out of memory\n");
> > > +			else
> > > +				dev_warn(&dev->dev,
> > > +					 "Unexpected TX queue failure: %d\n",
> > > +					 capacity);
> > >  		}
> > >  		return NETDEV_TX_BUSY;
> > >  	}
> > 
> > It is not clear to me how xmit_skb() can return -ENOMEM.
> > xmit_skb() calls virtqueue_add_buf_gfp() which can return -ENOSPC.
> > Even vring_add_indirect() doesn't return -ENOMEM on kmalloc failure.
> 
> It makes more sense to have the device increment tx_droppped, 
> and return NETDEV_TX_OK. Skip the message (or make it a pr_debug()). 
> Network devices do not guarantee packet delivery, and if out of
> resources then holding more data in the
> queue is going to hurt not help the situation.
> 
> -- 

Well, I only keep the existing behaviour around.  The changes you propose
would be 2.6.36 material.
I have it on my todo list to look for a way to test performance under
GFP_ATOMIC failure scenario. Any suggestions?

-- 
MST