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

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

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.

>
> 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 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.

Thanks.