Re: [PATCH net v8 1/3] net: sched: fix packet stuck problem for lockless qdisc

 struct qdisc_size_table {

@@ -159,8 +160,33 @@ static inline bool qdisc_is_empty(const struct Qdisc *qdisc)
 static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 {
        if (qdisc->flags & TCQ_F_NOLOCK) {
+               if (spin_trylock(&qdisc->seqlock))
+                       goto nolock_empty;
+
+               /* If the MISSED flag is set, it means other thread has
+                * set the MISSED flag before second spin_trylock(), so
+                * we can return false here to avoid multi cpus doing
+                * the set_bit() and second spin_trylock() concurrently.
+                */
+               if (test_bit(__QDISC_STATE_MISSED, &qdisc->state))
+                       return false;
+
+               /* Set the MISSED flag before the second spin_trylock(),
+                * if the second spin_trylock() return false, it means
+                * other cpu holding the lock will do dequeuing for us
+                * or it will see the MISSED flag set after releasing
+                * lock and reschedule the net_tx_action() to do the
+                * dequeuing.
+                */
+               set_bit(__QDISC_STATE_MISSED, &qdisc->state);
+
+               /* Retry again in case other CPU may not see the new flag
+                * after it releases the lock at the end of qdisc_run_end().
+                */
                if (!spin_trylock(&qdisc->seqlock))
                        return false;
+
+nolock_empty:
                WRITE_ONCE(qdisc->empty, false);
        } else if (qdisc_is_running(qdisc)) {
                return false;

@@ -176,8 +202,15 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
        write_seqcount_end(&qdisc->running);
-       if (qdisc->flags & TCQ_F_NOLOCK)
+       if (qdisc->flags & TCQ_F_NOLOCK) {
                spin_unlock(&qdisc->seqlock);
+
+               if (unlikely(test_bit(__QDISC_STATE_MISSED,
+                                     &qdisc->state))) {
+                       clear_bit(__QDISC_STATE_MISSED, &qdisc->state);

+                       __netif_schedule(qdisc);
+               }
+       }
 }

 static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 44991ea..795d986 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c

@@ -640,8 +640,10 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
 {
        struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
        struct sk_buff *skb = NULL;
+       bool need_retry = true;
        int band;

+retry:
        for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) {
                struct skb_array *q = band2list(priv, band);

@@ -652,6 +654,23 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
        }
        if (likely(skb)) {
                qdisc_update_stats_at_dequeue(qdisc, skb);
+       } else if (need_retry &&
+                  test_bit(__QDISC_STATE_MISSED, &qdisc->state)) {
+               /* Delay clearing the STATE_MISSED here to reduce
+                * the overhead of the second spin_trylock() in
+                * qdisc_run_begin() and __netif_schedule() calling
+                * in qdisc_run_end().
+                */
+               clear_bit(__QDISC_STATE_MISSED, &qdisc->state);