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@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html