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

From: Yunsheng Lin <linyunsheng@huawei.com>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	"Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>,
	Jakub Kicinski <kuba@kernel.org>,
	David Miller <davem@davemloft.net>,
	Vladimir Oltean <olteanv@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andriin@fb.com>,
	Eric Dumazet <edumazet@google.com>,
	"Wei Wang" <weiwan@google.com>,
	"Cong Wang ." <cong.wang@bytedance.com>,
	Taehee Yoo <ap420073@gmail.com>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>, <linuxarm@openeuler.org>,
	Marc Kleine-Budde <mkl@pengutronix.de>,
	<linux-can@vger.kernel.org>
Subject: Re: [Linuxarm] Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc
Date: Thu, 18 Mar 2021 15:33:04 +0800	[thread overview]
Message-ID: <3bae7b26-9d7f-15b8-d466-ff5c26d08b35@huawei.com> (raw)
In-Reply-To: <CAHmME9qDU7VRmBV+v0tzLiUpMJykjswSDwqc9P43ZwG1UD7mzw@mail.gmail.com>

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.

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:)

1.https://patchwork.kernel.org/project/netdevbpf/patch/1616050402-37023-1-git-send-email-linyunsheng@huawei.com/

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