linux-kernel - Re: [RFC v2] net: sched: implement TCQ_F_CAN

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Message-ID: <87eegddhsj.fsf@toke.dk>
Date:   Wed, 17 Mar 2021 14:35:40 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Yunsheng Lin <linyunsheng@...wei.com>,
        David Miller <davem@...emloft.net>,
        Vladimir Oltean <olteanv@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        "Cong Wang ." <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linuxarm@...neuler.org,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        linux-can@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

Cong Wang <xiyou.wangcong@...il.com> writes:

> On Mon, Mar 15, 2021 at 2:07 PM Jakub Kicinski <kuba@...nel.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 :)

-Toke