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SubjectRe: [PATCH RFC 3/3] virtio_net: limit xmit polling
On Thu, Jun 02, 2011 at 01:24:57PM +0930, Rusty Russell wrote:
> On Wed, 1 Jun 2011 12:50:03 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > Current code might introduce a lot of latency variation
> > if there are many pending bufs at the time we
> > attempt to transmit a new one. This is bad for
> > real-time applications and can't be good for TCP either.
> >
> > Free up just enough to both clean up all buffers
> > eventually and to be able to xmit the next packet.
>
> OK, I found this quite confusing to read.
>
> > - while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) {
> > + while ((r = virtqueue_min_capacity(vi->svq) < MAX_SKB_FRAGS + 2) ||
> > + min_skbs-- > 0) {
> > + skb = virtqueue_get_buf(vi->svq, &len);
> > + if (unlikely(!skb))
> > + 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;
> > }
>
> Gah... what a horrible loop.
>
> Basically, this patch makes hard-to-read code worse, and we should try
> to make it better.
>
> Currently, xmit *can* fail when an xmit interrupt wakes the queue, but
> the packet(s) xmitted didn't free up enough space for the new packet.
> With indirect buffers this only happens if we hit OOM (and thus go to
> direct buffers).
>
> We could solve this by only waking the queue in skb_xmit_done if the
> capacity is >= 2 + MAX_SKB_FRAGS. But can we do it without a race?

I don't think so.

> If not, then I'd really prefer to see this, because I think it's clearer:
>
> // Try to free 2 buffers for every 1 xmit, to stay ahead.
> free_old_buffers(2)
>
> if (!add_buf()) {
> // Screw latency, free them all.
> free_old_buffers(UINT_MAX)
> // OK, this can happen if we are using direct buffers,
> // and the xmit interrupt woke us but the packets
> // xmitted were smaller than this one. Rare though.
> if (!add_buf())
> Whinge and stop queue, maybe loop.
> }
>
> if (capacity < 2 + MAX_SKB_FRAGS) {
> // We don't have enough for the next packet? Try
> // freeing more.
> free_old_buffers(UINT_MAX);
> if (capacity < 2 + MAX_SKB_FRAGS) {
> Stop queue, maybe loop.
> }
>
> The current code makes my head hurt :(
>
> Thoughts?
> Rusty.

OK, I have something very similar, but I still dislike the screw the
latency part: this path is exactly what the IBM guys seem to hit. So I
created two functions: one tries to free a constant number and another
one up to capacity. I'll post that now.


--
MST


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