Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

[Kehuan Feng <kehuan.feng@gmail.com>]

Hi Hillf,

Unfortunately, above mem barriers don't help. The issue shows up
within 1 minute ...

Hillf Danton <hdanton@sina.com> 于2020年8月27日周四 下午8:58写道：

>
>
> On Thu, 27 Aug 2020 14:56:31 +0800 Kehuan Feng wrote:
> >
> > > Lets see if TCQ_F_NOLOC is making fq_codel different in your testing.
> >
> > I assume you meant disabling NOLOCK for pfifo_fast.
> >
> > Here is the modification,
> >
> > --- ./net/sched/sch_generic.c.orig      2020-08-24 22:02:04.589830751 +0800
> > +++ ./net/sched/sch_generic.c   2020-08-27 10:17:10.148977195 +0800
> > @@ -792,7 +792,7 @@
> >         .dump           =3D       pfifo_fast_dump,
> >         .change_tx_queue_len =3D  pfifo_fast_change_tx_queue_len,
> >         .owner          =3D       THIS_MODULE,
> > -       .static_flags   =3D       TCQ_F_NOLOCK | TCQ_F_CPUSTATS,
> > +       .static_flags   =3D       TCQ_F_CPUSTATS,
> >
> > The issue never happen again with it for over 3 hours stressing. And I
> > restarted the test for two times. No any surprising. Quite stable...
>
> Jaw off. That is great news and I'm failing again to explain the test
> result wrt the difference TCQ_F_NOLOCK can make in running qdisc.
>
> Nothing comes into mind other than two mem barriers though only one is
> needed...
>
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3040,6 +3040,7 @@ static void __netif_reschedule(struct Qd
>
>  void __netif_schedule(struct Qdisc *q)
>  {
> +       smp_mb__before_atomic();
>         if (!test_and_set_bit(__QDISC_STATE_SCHED, &q->state))
>                 __netif_reschedule(q);
>  }
> @@ -4899,6 +4900,7 @@ static __latent_entropy void net_tx_acti
>                          */
>                         smp_mb__before_atomic();
>                         clear_bit(__QDISC_STATE_SCHED, &q->state);
> +                       smp_mb__after_atomic();
>                         qdisc_run(q);
>                         if (root_lock)
>                                 spin_unlock(root_lock);
>