netdev - Re: "lockless" qdisc breaks tx_queue

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Date:   Wed, 3 Jan 2018 15:41:27 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: "lockless" qdisc breaks tx_queue_len change too?

On Wed, Jan 3, 2018 at 10:09 AM, John Fastabend
<john.fastabend@...il.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.