Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

[Yunsheng Lin]

On 2020/9/11 4:19, Cong Wang wrote:
> On Thu, Sep 3, 2020 at 8:21 PM Kehuan Feng <kehuan.feng@xxxxxxxxx> 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.
>