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

[Yunsheng Lin <linyunsheng@huawei.com>]

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 .

For enqueuing, multiple consumers is protected by producer_lock, see
pfifo_fast_enqueue() -> skb_array_produce() -> ptr_ring_produce().
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.

> 
> On the other hand, I don't think the problems we have been fixing are the ring
> buffer implementation itself, they are about the high-level qdisc
> state transitions.
> 
> Thanks.
> 
> .
>