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

[Cong Wang <xiyou.wangcong@gmail.com>]

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.

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.