On Mon, May 23, 2011 at 11:37:15AM +0930, Rusty Russell wrote: > On Sun, 22 May 2011 15:10:08 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > On Sat, May 21, 2011 at 11:49:59AM +0930, Rusty Russell wrote: > > > On Fri, 20 May 2011 02:11:56 +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. > > > > > > Do we have more than speculation to back that up, BTW? > > > > Need to dig this up: I thought we saw some reports of this on the list? > > I think so too, but a reference needs to be here too. > > It helps to have exact benchmarks on what's being tested, otherwise we > risk unexpected interaction with the other optimization patches. > > > > > struct sk_buff *skb; > > > > unsigned int len; > > > > - > > > > - while ((skb = virtqueue_get_buf(vi->svq, &len)) != NULL) { > > > > + bool c; > > > > + int n; > > > > + > > > > + /* We try to free up at least 2 skbs per one sent, so that we'll get > > > > + * all of the memory back if they are used fast enough. */ > > > > + for (n = 0; > > > > + ((c = virtqueue_get_capacity(vi->svq) < capacity) || n < 2) && > > > > + ((skb = virtqueue_get_buf(vi->svq, &len))); > > > > + ++n) { > > > > 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 !c; > > > > > > This is for() abuse :) > > > > > > Why is the capacity check in there at all? Surely it's simpler to try > > > to free 2 skbs each time around? > > > > This is in case we can't use indirect: we want to free up > > enough buffers for the following add_buf to succeed. > > Sure, or we could just count the frags of the skb we're taking out, > which would be accurate for both cases and far more intuitive. > > ie. always try to free up twice as much as we're about to put in. > > Can we hit problems with OOM? Sure, but no worse than now... > The problem is that this "virtqueue_get_capacity()" returns the worst > case, not the normal case. So using it is deceptive. > Maybe just document this? I still believe capacity really needs to be decided at the virtqueue level, not in the driver. E.g. with indirect each skb uses a single entry: freeing 1 small skb is always enough to have space for a large one. I do understand how it seems a waste to leave direct space in the ring while we might in practice have space due to indirect. Didn't come up with a nice way to solve this yet - but 'no worse than now :)' > > I just wanted to localize the 2+MAX_SKB_FRAGS logic that tries to make > > sure we have enough space in the buffer. Another way to do > > that is with a define :). > > 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? -- 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