linux-kernel - Re: Packet gets stuck in NOLOCK pfifo

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Date:   Thu, 10 Sep 2020 13:19:01 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Kehuan Feng <kehuan.feng@...il.com>
Cc:     Hillf Danton <hdanton@...a.com>, Paolo Abeni <pabeni@...hat.com>,
        Jike Song <albcamus@...il.com>, Josh Hunt <johunt@...mai.com>,
        Jonas Bonn <jonas.bonn@...rounds.com>,
        Michael Zhivich <mzhivich@...mai.com>,
        David Miller <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

On Thu, Sep 3, 2020 at 8:21 PM Kehuan Feng <kehuan.feng@...il.com> wrote:
> I also tried Cong's patch (shown below on my tree) and it could avoid
> the issue (stressing for 30 minutus for three times and not jitter
> observed).

Thanks for verifying it!

>
> --- ./include/net/sch_generic.h.orig 2020-08-21 15:13:51.787952710 +0800
> +++ ./include/net/sch_generic.h 2020-09-03 21:36:11.468383738 +0800
> @@ -127,8 +127,7 @@
>  static inline bool qdisc_run_begin(struct Qdisc *qdisc)
>  {
>   if (qdisc->flags & TCQ_F_NOLOCK) {
> - if (!spin_trylock(&qdisc->seqlock))
> - return false;
> + spin_lock(&qdisc->seqlock);
>   } else if (qdisc_is_running(qdisc)) {
>   return false;
>   }
>
> I am not actually know what you are discussing above. It seems to me
> that Cong's patch is similar as disabling lockless feature.

>From performance's perspective, yeah. Did you see any performance
downgrade with my patch applied? It would be great if you can compare
it with removing NOLOCK. And if the performance is as bad as no
NOLOCK, then we can remove the NOLOCK bit for pfifo_fast, at least
for now.

Thanks.