Re: [PATCH RFC 2/3] virtio_net: fix tx capacity checks using new API

On Thu, Jun 02, 2011 at 11:40:26AM +0930, Rusty Russell wrote:

On Wed, 1 Jun 2011 12:49:54 +0300, "Michael S. Tsirkin" [off-list ref] wrote:

quoted

In the (rare) case where new descriptors are used
while virtio_net enables vq callback for the TX vq,
virtio_net uses the number of sg entries in the skb it frees to
calculate how many descriptors in the ring have just been made
available.  But this value is an overestimate: with indirect buffers
each skb only uses one descriptor entry, meaning we may wake the queue
only to find we still can't transmit anything.

This is a bit misleading.

The value is an overestimate, but so is the requirement for
2+MAX_SKB_FRAGS, *unless* we suddenly drop into direct mode due to OOM.

Thanks,
Rusty.

I agree, it's unlikely.

s/still can't transmit anything/are still out of space and need to stop
the ring almost at once/

Better?