On Thu, Jun 02, 2011 at 08:56:42PM +0530, Krishna Kumar2 wrote:
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote on 06/02/2011 08:13:46 PM:
>
> > > Please review this patch to see if it looks reasonable:
> >
> > Hmm, since you decided to work on top of my patch,
> > I'd appreciate split-up fixes.
>
> OK (that also explains your next comment).
>
> > > 1. Picked comments/code from MST's code and Rusty's review.
> > > 2. virtqueue_min_capacity() needs to be called only if it returned
> > > empty the last time it was called.
> > > 3. Fix return value bug in free_old_xmit_skbs (hangs guest).
> > > 4. Stop queue only if capacity is not enough for next xmit.
> >
> > That's what we always did ...
>
> I had made the patch against your patch, hence this change (sorry for
> the confusion!).
>
> > > 5. Fix/clean some likely/unlikely checks (hopefully).
> > >
> > > I have done some minimal netperf tests with this.
> > >
> > > With this patch, add_buf returning capacity seems to be useful - it
> > > allows less virtio API calls.
> >
> > Why bother? It's cheap ...
>
> If add_buf retains it's functionality to return the capacity (it
> is going to need a change to return 0 otherwise anyway), is it
> useful to call another function at each xmit?
>
> > > +static bool free_old_xmit_skbs(struct virtnet_info *vi, int to_free)
> > > +{
> > > + bool empty = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2;
> > > +
> > > + do {
> > > + if (!free_one_old_xmit_skb(vi)) {
> > > + /* No more skbs to free up */
> > > break;
> > > - pr_debug("Sent skb %p\n", skb);
> > > - vi->dev->stats.tx_bytes += skb->len;
> > > - vi->dev->stats.tx_packets++;
> > > - dev_kfree_skb_any(skb);
> > > - }
> > > - return r;
> > > + }
> > > +
> > > + if (empty) {
> > > + /* Check again if there is enough space */
> > > + empty = virtqueue_min_capacity(vi->svq) <
> > > + MAX_SKB_FRAGS + 2;
> > > + } else {
> > > + --to_free;
> > > + }
> > > + } while (to_free > 0);
> > > +
> > > + return !empty;
> > > }
> >
> > Why bother doing the capacity check in this function?
>
> To return whether we have enough space for next xmit. It should call
> it only once unless space is running out. Does it sound OK?
>
> > > - if (unlikely(ret < 0)) {
> > > + if (unlikely(capacity < 0)) {
> > > + /*
> > > + * Failure to queue should be impossible. The only way to
> > > + * reach here is if we got a cb before 3/4th of space was
> > > + * available. We could stop the queue and re-enable
> > > + * callbacks (and possibly return TX_BUSY), but we don't
> > > + * bother since this is impossible.
> >
> > It's far from impossible. The 3/4 thing is only a hint, and old devices
> > don't support it anyway.
>
> OK, I will re-put back your comment.
>
> > > - if (!likely(free_old_xmit_skbs(vi, 2))) {
> > > - netif_stop_queue(dev);
> > > - if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > > - /* More just got used, free them and recheck. */
> > > - if (!likely(free_old_xmit_skbs(vi, 0))) {
> > > - netif_start_queue(dev);
> > > - virtqueue_disable_cb(vi->svq);
> > > + /*
> > > + * Apparently nice girls don't return TX_BUSY; check capacity and
> > > + * stop the queue before it gets out of hand. Naturally, this
> wastes
> > > + * entries.
> > > + */
> > > + if (capacity < 2+MAX_SKB_FRAGS) {
> > > + /*
> > > + * We don't have enough space for the next packet. Try
> > > + * freeing more.
> > > + */
> > > + if (likely(!free_old_xmit_skbs(vi, UINT_MAX))) {
> > > + netif_stop_queue(dev);
> > > + if (unlikely(!virtqueue_enable_cb_delayed(vi->svq))) {
> > > + /* More just got used, free them and recheck. */
> > > + if (likely(free_old_xmit_skbs(vi, UINT_MAX))) {
> >
> > Is this where the bug was?
>
> Return value in free_old_xmit() was wrong. I will re-do against the
> mainline kernel.
>
> Thanks,
>
> - KK
Just noting that I'm working on that patch as well, it might
be more efficient if we don't both of us do this in parallel :)
--
MST
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