On Wed, Jan 3, 2018 at 10:09 AM, John Fastabend <john.fastab...@gmail.com> wrote: > On 01/02/2018 08:41 PM, Cong Wang wrote: >> Hi, John >> >> While reviewing your ptr_ring fix again today, it looks like your >> "lockless" qdisc patchset breaks dev->tx_queue_len behavior. >> >> Before your patchset, dev->tx_queue_len is merely an integer to read, >> after your patchset, the skb array has to be resized when >> dev->tx_queue_len changes, but I don't see any qdisc code handles >> this... >> >> Also, because of that, I doubt __skb_array_empty() in >> pfifo_fast_dequeue() can be safe any more even with your ptr_ring fix. >> >> What am I missing? >> > > I dropped support for tx_queue_len changes after qdisc has been > created. The only check is at init time when building the qdisc.
This is where it breaks. > > Before this series teql and pfifo_fast were the only qdiscs that > used tx_queue_len other qdiscs used other mechanisms or copied > tx_queue_len at init time. So the API is inconsistent. Yeah, pfifo_fast was able to drop based on latest value of tx_queue_len before your patchset, this is why I am complaining. > > OK, but arguably its kAPI now and needs to be supported on live > qdiscs. So couple options drop the __skb_array_empty() check, > stop supporting changes on running qdiscs, or do a qdisc swap > with the new array. I don't think we can break the old behavior of tx_queue_len change for pfifo_fast, people may already rely on it. Doing a swap seems reasonable. > > I'm tempted to make the qdisc swap work, still need benchmarks > I guess without the empty check. Either way to get it working > we need a callback from tx_queue_len code paths. Right, probably need a new ops in Qdisc_ops. > > Unfortunately, I guess someone somewhere probably uses pfifo_fast > and changes there queue length with a script after creating the > qdisc and expects it to work. > This is my concern as well. I will work on some patches, this doesn't look trivial to solve at all. Thanks.
