netdev - Re: [Patch net-next v3 0/3] net_sched: reflect tx_queue_len change for pfifo

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Message-ID: <CAM_iQpVgDGjfyRU-G_tE5AvLsnVZW+yANjAPkxyB5hrKnuhjQw@mail.gmail.com>
Date:   Sun, 28 Jan 2018 22:25:05 -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: [Patch net-next v3 0/3] net_sched: reflect tx_queue_len change
 for pfifo_fast

On Sun, Jan 28, 2018 at 10:09 PM, John Fastabend
<john.fastabend@...il.com> wrote:
> On 01/28/2018 09:57 PM, Cong Wang wrote:
>> On Sun, Jan 28, 2018 at 9:35 PM, John Fastabend
>> <john.fastabend@...il.com> wrote:
>>> On 01/25/2018 06:26 PM, Cong Wang wrote:
>>>> This pathcset restores the pfifo_fast qdisc behavior of dropping
>>>> packets based on latest dev->tx_queue_len. Patch 1 introduces
>>>> a helper, patch 2 introduces a new Qdisc ops which is called when
>>>> we modify tx_queue_len, patch 3 implements this ops for pfifo_fast.
>>>>
>>>> Please see each patch for details.
>>>>
>>>
>>> Overall this series is better than what we have at the moment, but
>>> a better fix would preallocate the memory, to avoid ENOMEM errors,
>>
>> I am not against for better ENOMEM error handling, but I still have to
>> remind you that attach_one_default_qdisc() doesn't handle it either.
>> Since no one complained about it, why this one is so special?
>
> Its not we should fix both cases. And also clean up the multiple
> net sync calls in these paths as well.

Now can we agree error handling can be improved later? You
already agree this is not a new problem introduced by this patchset,
why do you want to block a regression fix just because of an old
problem which I will fix later?


>
>>
>>
>>> and add a ptr_ring API to use the preallocated buffers.
>>
>>
>> What ptr_ring API could cure netdev tx queues problem here?
>> Looks like you still don't understand the problem here.
>>
>
> The resize multiple array API can only fail due to alloc failures.
> We need to break this API into two pieces preallocate the memory
> and then commit array changes. Alternatively the qdisc layer could
> allocate new arrays and then swap them after all the arrays been
> initialized correctly using ptr_ring APIs below the ptr_ring
> resize API calls.
>
> Having ptr_ring API operations to support this seems best.
>


Apparently qdisc layer doesn't care about ptr_ring, as you describe
here this potentially needs to change two layers: 1) qdisc layer
2) ptr_ring API. It is more work than just a simple error handling,
potentially bigger than this patchset itself.


>>
>>>
>>> We have time (its only in net-next) so lets do the complete fix
>>> rather than this series IMO.
>>>
>>
>> Why not just accept this and complete the error handling later
>> given the fact that I already add a TODO? IOW, why it is now?
>>
>
> Because its not correct and its not too much work to get it so
> the error is avoided.

So you must want a fix for net too, because attach_one_default_qdisc()
"is not correct" either and "it is not too much work".

I can't agree on any of your claims here. Sorry.