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Date: Mon, 14 Sep 2020 10:10:31 +0800 From: Yunsheng Lin <linyunsheng@...wei.com> To: Cong Wang <xiyou.wangcong@...il.com>, 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>, "linuxarm@...wei.com" <linuxarm@...wei.com> Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc On 2020/9/11 4:19, Cong Wang wrote: > 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. It seems the lockless qdisc may have below concurrent problem: cpu0: cpu1: q->enqueue . qdisc_run_begin(q) . __qdisc_run(q) ->qdisc_restart() -> dequeue_skb() . -> sch_direct_xmit() . . q->enqueue qdisc_run_begin(q) qdisc_run_end(q) cpu1 enqueue a skb without calling __qdisc_run(), and cpu0 did not see the enqueued skb when calling __qdisc_run(q) because cpu1 may enqueue the skb after cpu0 called __qdisc_run(q) and before cpu0 called qdisc_run_end(q). Kehuan, do you care to try the below patch if it is the same problem? diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h index d60e7c3..c97c1ed 100644 --- a/include/net/sch_generic.h +++ b/include/net/sch_generic.h @@ -36,6 +36,7 @@ struct qdisc_rate_table { enum qdisc_state_t { __QDISC_STATE_SCHED, __QDISC_STATE_DEACTIVATED, + __QDISC_STATE_ENQUEUED, }; struct qdisc_size_table { diff --git a/net/core/dev.c b/net/core/dev.c index 0362419..5985648 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3748,6 +3748,8 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q, qdisc_calculate_pkt_len(skb, q); if (q->flags & TCQ_F_NOLOCK) { + set_bit(__QDISC_STATE_ENQUEUED, &q->state); + smp_mb__after_atomic(); rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK; qdisc_run(q); diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index 265a61d..c389641 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -381,6 +381,8 @@ void __qdisc_run(struct Qdisc *q) int quota = dev_tx_weight; int packets; + clear_bit(__QDISC_STATE_ENQUEUED, &q->state); + smp_mb__after_atomic(); while (qdisc_restart(q, &packets)) { quota -= packets; if (quota <= 0) { @@ -388,6 +390,9 @@ void __qdisc_run(struct Qdisc *q) break; } } + + if (test_bit(__QDISC_STATE_ENQUEUED, &q->state)) + __netif_schedule(q); } unsigned long dev_trans_start(struct net_device *dev) > > Thanks. >
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