Re: [PATCHv2 10/14] virtio_net: limit xmit polling

Michael S. Tsirkin
2011-05-24T09:12:55Z

On Tue, May 24, 2011 at 01:24:15PM +0530, Krishna Kumar2 wrote:
> "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote on 05/23/2011 04:49:00 PM:
>
> > > To do this properly, we should really be using the actual number of sg
> > > elements needed, but we'd have to do most of xmit_skb beforehand so we
> > > know how many.
> > >
> > > Cheers,
> > > Rusty.
> >
> > Maybe I'm confused here. The problem isn't the failing
> > add_buf for the given skb IIUC. What we are trying to do here is stop
> > the queue *before xmit_skb fails*. We can't look at the
> > number of fragments in the current skb - the next one can be
> > much larger. That's why we check capacity after xmit_skb,
> > not before it, right?
>
> Maybe Rusty means it is a simpler model to free the amount
> of space that this xmit needs. We will still fail anyway
> at some time but it is unlikely, since earlier iteration
> freed up atleast the space that it was going to use.

Not sure I nderstand. We can't know space is freed in the previous
iteration as buffers might not have been used by then.

> The
> code could become much simpler:
>
> start_xmit()
> {
> {
> num_sgs = get num_sgs for this skb;
>
> /* Free enough pending old buffers to enable queueing this one */
> free_old_xmit_skbs(vi, num_sgs * 2); /* ?? */
>
> if (virtqueue_get_capacity() < num_sgs) {
> netif_stop_queue(dev);
> if (virtqueue_enable_cb_delayed(vi->svq) ||
> free_old_xmit_skbs(vi, num_sgs)) {
> /* Nothing freed up, or not enough freed up */
> kfree_skb(skb);
> return NETDEV_TX_OK;

This packet drop is what we wanted to avoid.

> }
> netif_start_queue(dev);
> virtqueue_disable_cb(vi->svq);
> }
>
> /* xmit_skb cannot fail now, also pass 'num_sgs' */
> xmit_skb(vi, skb, num_sgs);
> virtqueue_kick(vi->svq);
>
> skb_orphan(skb);
> nf_reset(skb);
>
> return NETDEV_TX_OK;
> }
>
> We could even return TX_BUSY since that makes the dequeue
> code more efficient. See dev_dequeue_skb() - you can skip a
> lot of code (and avoid taking locks) to check if the queue
> is already stopped but that code runs only if you return
> TX_BUSY in the earlier iteration.
>
> BTW, shouldn't the check in start_xmit be:
> if (likely(!free_old_xmit_skbs(vi, 2+MAX_SKB_FRAGS))) {
> ...
> }
>
> Thanks,
>
> - KK

I thought we used to do basically this but other devices moved to a
model where they stop *before* queueing fails, so we did too.

--
MST