Re: "lockless" qdisc breaks tx_queue_len change too?

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.