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Message-ID: <CACS=qqKhsu6waaXndO5tQL_gC9TztuUQpqQigJA2Ac0y12czMQ@mail.gmail.com>
Date: Tue, 25 Aug 2020 10:18:05 +0800
From: Fengkehuan Feng <kehuan.feng@...il.com>
To: Hillf Danton <hdanton@...a.com>
Cc: Jike Song <albcamus@...il.com>, Josh Hunt <johunt@...mai.com>,
Paolo Abeni <pabeni@...hat.com>,
Jonas Bonn <jonas.bonn@...rounds.com>,
Cong Wang <xiyou.wangcong@...il.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
Hillf,
With the latest version (attached what I have changed on my tree), the
system failed to start up with cpu stalled.
Hillf Danton <hdanton@...a.com> 于2020年8月22日周六 上午11:30写道：
>
>
> On Thu, 20 Aug 2020 20:43:17 +0800 Hillf Danton wrote:
> > Hi Jike,
> >
> > On Thu, 20 Aug 2020 15:43:17 +0800 Jike Song wrote:
> > > Hi Josh,
> > >
> > > On Fri, Jul 3, 2020 at 2:14 AM Josh Hunt <johunt@...mai.com> wrote:
> > > {snip}
> > > > Initial results with Cong's patch look promising, so far no stalls. We
> > > > will let it run over the long weekend and report back on Tuesday.
> > > >
> > > > Paolo - I have concerns about possible performance regression with the
> > > > change as well. If you can gather some data that would be great. If
> > > > things look good with our low throughput test over the weekend we can
> > > > also try assessing performance next week.
> > > >
> > >
> > > We met possibly the same problem when testing nvidia/mellanox's
> >
> > Below is what was sent in reply to this thread early last month with
> > minor tuning, based on the seqlock. Feel free to drop an echo if it
> > makes ant-antenna-size sense in your tests.
> >
> > > GPUDirect RDMA product, we found that changing NET_SCH_DEFAULT to
> > > DEFAULT_FQ_CODEL mitigated the problem, having no idea why. Maybe you
> > > can also have a try?
> > >
> > > Besides, our testing is pretty complex, do you have a quick test to
> > > reproduce it?
> > >
> > > --
> > > Thanks,
> > > Jike
> >
> >
> > --- a/include/net/sch_generic.h
> > +++ b/include/net/sch_generic.h
> > @@ -79,6 +79,7 @@ struct Qdisc {
> > #define TCQ_F_INVISIBLE 0x80 /* invisible by default in dump */
> > #define TCQ_F_NOLOCK 0x100 /* qdisc does not require locking */
> > #define TCQ_F_OFFLOADED 0x200 /* qdisc is offloaded to HW */
> > + int pkt_seq;
> > u32 limit;
> > const struct Qdisc_ops *ops;
> > struct qdisc_size_table __rcu *stab;
> > @@ -156,6 +157,7 @@ static inline bool qdisc_is_empty(const
> > static inline bool qdisc_run_begin(struct Qdisc *qdisc)
> > {
> > if (qdisc->flags & TCQ_F_NOLOCK) {
> > + qdisc->pkt_seq++;
> > if (!spin_trylock(&qdisc->seqlock))
> > return false;
> > WRITE_ONCE(qdisc->empty, false);
> > --- a/include/net/pkt_sched.h
> > +++ b/include/net/pkt_sched.h
> > @@ -117,7 +117,9 @@ void __qdisc_run(struct Qdisc *q);
> >
> > static inline void qdisc_run(struct Qdisc *q)
> > {
> > - if (qdisc_run_begin(q)) {
> > + while (qdisc_run_begin(q)) {
> > + int seq = q->pkt_seq;
> > +
> > /* NOLOCK qdisc must check 'state' under the qdisc seqlock
> > * to avoid racing with dev_qdisc_reset()
> > */
> > @@ -125,6 +127,9 @@ static inline void qdisc_run(struct Qdis
> > likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
> > __qdisc_run(q);
> > qdisc_run_end(q);
> > +
> > + if (!(q->flags & TCQ_F_NOLOCK) || seq == q->pkt_seq)
> > + return;
> > }
> > }
>
> The echo from Feng indicates that it's hard to conclude that TCQ_F_NOLOCK
> is the culprit, lets try again with it ignored for now.
>
> Every pkt enqueued on pfifo_fast is tracked in the below diff, and those
> pkts enqueued while we're running qdisc are detected and handled to cut
> the chance for the stuck pkts reported.
>
> --- a/include/net/sch_generic.h
> +++ b/include/net/sch_generic.h
> @@ -79,6 +79,7 @@ struct Qdisc {
> #define TCQ_F_INVISIBLE 0x80 /* invisible by default in dump */
> #define TCQ_F_NOLOCK 0x100 /* qdisc does not require locking */
> #define TCQ_F_OFFLOADED 0x200 /* qdisc is offloaded to HW */
> + int pkt_seq;
> u32 limit;
> const struct Qdisc_ops *ops;
> struct qdisc_size_table __rcu *stab;
> --- a/net/sched/sch_generic.c
> +++ b/net/sched/sch_generic.c
> @@ -631,6 +631,7 @@ static int pfifo_fast_enqueue(struct sk_
> return qdisc_drop(skb, qdisc, to_free);
> }
>
> + qdisc->pkt_seq++;
> qdisc_update_stats_at_enqueue(qdisc, pkt_len);
> return NET_XMIT_SUCCESS;
> }
> --- a/include/net/pkt_sched.h
> +++ b/include/net/pkt_sched.h
> @@ -117,7 +117,8 @@ void __qdisc_run(struct Qdisc *q);
>
> static inline void qdisc_run(struct Qdisc *q)
> {
> - if (qdisc_run_begin(q)) {
> + while (qdisc_run_begin(q)) {
> + int seq = q->pkt_seq;
> /* NOLOCK qdisc must check 'state' under the qdisc seqlock
> * to avoid racing with dev_qdisc_reset()
> */
> @@ -125,6 +126,12 @@ static inline void qdisc_run(struct Qdis
> likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
> __qdisc_run(q);
> qdisc_run_end(q);
> +
> + /* go another round if there are pkts enqueued after
> + * taking seq_lock
> + */
> + if (seq != q->pkt_seq)
> + continue;
> }
> }
>
>
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