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

On Thu, May 13, 2021 at 7:27 PM Yunsheng Lin linyunsheng@huawei.com wrote:

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);
  

We have test_and_clear_bit() which is atomic, test_bit()+clear_bit() is not.

•                   __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);
  

Ditto.

•           /* Make sure dequeuing happens after clearing
  

•            * STATE_MISSED.
  

•            */
  

•           smp_mb__after_atomic();
  

•           need_retry = false;
  

•           goto retry;
  

Two concurrent pfifo_fast_dequeue() would possibly retry it at the same time when they test __QDISC_STATE_MISSED at the same time and get true. Is this a problem?

Also, any reason why you want pfifo_fast to handle a generic Qdisc flag? IOW, why not handle this logic in, for example, qdisc_restart()?

Thanks.