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

From: Yunsheng Lin
Date: Thu Mar 18 2021 - 03:34:38 EST

On 2021/3/17 21:45, Jason A. Donenfeld wrote:
> On 3/17/21, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>> Cong Wang <xiyou.wangcong@xxxxxxxxx> writes:
>>> On Mon, Mar 15, 2021 at 2:07 PM Jakub Kicinski <kuba@xxxxxxxxxx> 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.

But right now lockless qdisc has some packet stuck problem, which I tried to
fix in [1].

If the packet stuck problem for lockless qdisc can be fixed, and we can do
more optimization on lockless qdisc, including the one you mention:)


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