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. 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. 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'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. 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. > Thanks. >
