Re: "lockless" qdisc breaks tx_queue_len change too?

On 01/03/2018 03:41 PM, Cong Wang wrote:

On Wed, Jan 3, 2018 at 10:09 AM, John Fastabend
[off-list ref] wrote:

quoted

On 01/02/2018 08:41 PM, Cong Wang wrote:

quoted

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.

quoted

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.

Yep good complaint.

quoted

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.

Agreed needed for legacy support.

Doing a swap seems reasonable.

quoted

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.

Maybe instead of a Qdisc op just do a direct call to avoid
encouraging users to use this code path. Either way is
probably fine we can just watch any future patches and
have users add a specific attribute for it like codel.

quoted

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.

How about a dev_deactivate_many() that instead of replacing
with noop qdisc replaces with new updated qdisc. Seems like
it might work.

Thanks,
John

Thanks.