Re: [Linuxarm] Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/3/24 9:49, Cong Wang wrote:
> On Sun, Mar 21, 2021 at 5:55 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> On 2021/3/20 2:15, Cong Wang wrote:
>>> On Thu, Mar 18, 2021 at 12:33 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>>>
>>>> On 2021/3/17 21:45, Jason A. Donenfeld wrote:
>>>>> On 3/17/21, Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>>>>> Cong Wang <xiyou.wangcong@gmail.com> writes:
>>>>>>
>>>>>>> On Mon, Mar 15, 2021 at 2:07 PM Jakub Kicinski <kuba@kernel.org> wrote:
>>>>>>>>
>>>>>>>> I thought pfifo was supposed to be "lockless" and this change
>>>>>>>> re-introduces a lock between producer and consumer, no?
>>>>>>>
>>>>>>> It has never been truly lockless, it uses two spinlocks in the ring
>>>>>>> buffer
>>>>>>> implementation, and it introduced a q->seqlock recently, with this patch
>>>>>>> now we have priv->lock, 4 locks in total. So our "lockless" qdisc ends
>>>>>>> up having more locks than others. ;) I don't think we are going to a
>>>>>>> right direction...
>>>>>>
>>>>>> Just a thought, have you guys considered adopting the lockless MSPC ring
>>>>>> buffer recently introduced into Wireguard in commit:
>>>>>>
>>>>>> 8b5553ace83c ("wireguard: queueing: get rid of per-peer ring buffers")
>>>>>>
>>>>>> Jason indicated he was willing to work on generalising it into a
>>>>>> reusable library if there was a use case for it. I haven't quite though
>>>>>> through the details of whether this would be such a use case, but
>>>>>> figured I'd at least mention it :)
>>>>>
>>>>> That offer definitely still stands. Generalization sounds like a lot of fun.
>>>>>
>>>>> Keep in mind though that it's an eventually consistent queue, not an
>>>>> immediately consistent one, so that might not match all use cases. It
>>>>> works with wg because we always trigger the reader thread anew when it
>>>>> finishes, but that doesn't apply to everyone's queueing setup.
>>>>
>>>> Thanks for mentioning this.
>>>>
>>>> "multi-producer, single-consumer" seems to match the lockless qdisc's
>>>> paradigm too, for now concurrent enqueuing/dequeuing to the pfifo_fast's
>>>> queues() is not allowed, it is protected by producer_lock or consumer_lock.
>>>>
>>>> So it would be good to has lockless concurrent enqueuing, while dequeuing
>>>> can be protected by qdisc_lock() or q->seqlock, which meets the "multi-producer,
>>>> single-consumer" paradigm.
>>>
>>> I don't think so. Usually we have one queue for each CPU so we can expect
>>> each CPU has a lockless qdisc assigned, but we can not assume this in
>>> the code, so we still have to deal with multiple CPU's sharing a lockless qdisc,
>>> and we usually enqueue and dequeue in process context, so it means we could
>>> have multiple producers and multiple consumers.
>>
>> For lockless qdisc, dequeuing is always within the qdisc_run_begin() and
>> qdisc_run_end(), so multiple consumers is protected with each other by
>> q->seqlock .
> 
> So are you saying you will never go lockless for lockless qdisc? I thought
> you really want to go lockless with Jason's proposal of MPMC ring buffer
> code.

I think we has different definition about lockless qdisc.

For my understanding, the dequeuing is within the qdisc_run_begin()
and qdisc_run_end(), so it is always protected by q->seqlock for
lockless qdisck currently, and by lockless qdisc, I never mean
lockless dequeuing, and I am not proposing lockless dequeuing
currently.

Current lockless qdisc for pfifo_fast only means there is no lock
for protection between dequeuing and enqueuing, which also means
when __qdisc_run() is dequeuing a skb while other cpu is enqueuing
a skb.

But enqueuing is protected by producer_lock in skb_array_produce(),
so only one cpu can do the enqueuing at the same time, so I am
proposing to use Jason's proposal to enable multi cpus to do
concurrent enqueuing without taking any lock.

> 
>>
>> For enqueuing, multiple consumers is protected by producer_lock, see
>> pfifo_fast_enqueue() -> skb_array_produce() -> ptr_ring_produce().
> 
> I think you seriously misunderstand how we classify MPMC or MPSC,
> it is not about how we lock them, it is about whether we truly have
> a single or multiple consumers regardless of locks used, because the
> goal is to go lockless.

I think I am only relying on the MPSC(multi-produce & single-consumer),
as explained above.

> 
>> I am not sure if lockless MSPC can work with the process context, but
>> even if not, the enqueuing is also protected by rcu_read_lock_bh(),
>> which provides some kind of atomicity, so that producer_lock can be
>> reomved when lockless MSPC is used.
> 
> 
> Not sure if I can even understand what you are saying here, Jason's
> code only disables preemption with busy wait, I can't see why it can
> not be used in the process context.

I am saying q->enqeue() is protected by rcu_read_lock_bh().
rcu_read_lock_bh() will disable preemption for us for most configuation,
otherwise it will break netdev_xmit_more() interface too, for it relies
on the cpu not being prempted by using per cpu var(softnet_data.xmit.more).

> 
> Thanks.
> 
> .
>