Re: [Linuxarm] Re: [PATCH net-next 2/3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

From: Jakub Kicinski <kuba@kernel.org>
To: Yunsheng Lin <linyunsheng@huawei.com>
Cc: Yunsheng Lin <yunshenglin0825@gmail.com>, <davem@davemloft.net>,
	<olteanv@gmail.com>, <ast@kernel.org>, <daniel@iogearbox.net>,
	<andriin@fb.com>, <edumazet@google.com>, <weiwan@google.com>,
	<cong.wang@bytedance.com>, <ap420073@gmail.com>,
	<netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linuxarm@openeuler.org>, <mkl@pengutronix.de>,
	<linux-can@vger.kernel.org>, <jhs@mojatatu.com>,
	<xiyou.wangcong@gmail.com>, <jiri@resnulli.us>,
	<andrii@kernel.org>, <kafai@fb.com>, <songliubraving@fb.com>,
	<yhs@fb.com>, <john.fastabend@gmail.com>, <kpsingh@kernel.org>,
	<bpf@vger.kernel.org>, <jonas.bonn@netrounds.com>,
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	<a.fatoum@pengutronix.de>, <atenart@kernel.org>,
	<alexander.duyck@gmail.com>, <hdanton@sina.com>,
	<jgross@suse.com>, <JKosina@suse.com>, <mkubecek@suse.cz>,
	<bjorn@kernel.org>, <alobakin@pm.me>
Subject: Re: [Linuxarm] Re: [PATCH net-next 2/3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc
Date: Tue, 1 Jun 2021 13:48:56 -0700	[thread overview]
Message-ID: <20210601134856.12573333@kicinski-fedora-PC1C0HJN.hsd1.ca.comcast.net> (raw)
In-Reply-To: <cf75e1f4-7972-8efa-7554-fc528c5da380@huawei.com>

On Tue, 1 Jun 2021 16:18:54 +0800 Yunsheng Lin wrote:
> > I see, thanks! That explains the need. Perhaps we can rephrase the
> > comment? Maybe:
> > 
> > +			/* Retest nolock_qdisc_is_empty() within the protection
> > +			 * of q->seqlock to protect from racing with requeuing.
> > +			 */  
> 
> Yes if we still decide to preserve the nolock_qdisc_is_empty() rechecking
> under q->seqlock.

Sounds good.

> >> --- a/net/sched/sch_generic.c
> >> +++ b/net/sched/sch_generic.c
> >> @@ -38,6 +38,15 @@ EXPORT_SYMBOL(default_qdisc_ops);
> >>  static void qdisc_maybe_clear_missed(struct Qdisc *q,
> >>                                      const struct netdev_queue *txq)
> >>  {
> >> +       set_bit(__QDISC_STATE_DRAINING, &q->state);
> >> +
> >> +       /* Make sure DRAINING is set before clearing MISSED
> >> +        * to make sure nolock_qdisc_is_empty() always return
> >> +        * false for aoviding transmitting a packet directly
> >> +        * bypassing the requeued packet.
> >> +        */
> >> +       smp_mb__after_atomic();
> >> +
> >>         clear_bit(__QDISC_STATE_MISSED, &q->state);
> >>
> >>         /* Make sure the below netif_xmit_frozen_or_stopped()
> >> @@ -52,8 +61,6 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q,
> >>          */
> >>         if (!netif_xmit_frozen_or_stopped(txq))
> >>                 set_bit(__QDISC_STATE_MISSED, &q->state);
> >> -       else
> >> -               set_bit(__QDISC_STATE_DRAINING, &q->state);
> >>  }  
> > 
> > But this would not be enough because we may also clear MISSING 
> > in pfifo_fast_dequeue()?  
> 
> For the MISSING clearing in pfifo_fast_dequeue(), it seems it
> looks like the data race described in RFC v3 too?
> 
>       CPU1                 CPU2               CPU3
> qdisc_run_begin(q)          .                  .
>         .              MISSED is set           .
>   MISSED is cleared         .                  .
>     q->dequeue()            .                  .
>         .              enqueue skb1     check MISSED # true
> qdisc_run_end(q)            .                  .
>         .                   .         qdisc_run_begin(q) # true
>         .            MISSED is set      send skb2 directly

Not sure what you mean.