netdev - Re: "lockless" qdisc breaks tx_queue

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Date:   Wed, 3 Jan 2018 19:03:30 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: 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
> <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.
> 

Yep good complaint.

> 
>>
>> 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.
> 
>>
>> 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.

> 
>>
>> 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.
>