Re: [PATCH net v6 3/3] net: sched: fix tx action reschedule issue with stopped queue

On 2021/5/11 12:22, Jakub Kicinski wrote:

On Mon, 10 May 2021 09:42:36 +0800 Yunsheng Lin wrote:

quoted

The netdev qeueue might be stopped when byte queue limit has
reached or tx hw ring is full, net_tx_action() may still be
rescheduled endlessly if STATE_MISSED is set, which consumes
a lot of cpu without dequeuing and transmiting any skb because
the netdev queue is stopped, see qdisc_run_end().

This patch fixes it by checking the netdev queue state before
calling qdisc_run() and clearing STATE_MISSED if netdev queue is
stopped during qdisc_run(), the net_tx_action() is recheduled
again when netdev qeueue is restarted, see netif_tx_wake_queue().

Fixes: 6b3ba9146fe6 ("net: sched: allow qdiscs to handle locking")
Reported-by: Michal Kubecek <redacted>
Signed-off-by: Yunsheng Lin <redacted>

Patches 1 and 2 look good to me but this one I'm not 100% sure.

quoted

@@ -251,8 +253,10 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
 	*validate = true;
 
 	if ((q->flags & TCQ_F_ONETXQUEUE) &&
-	    netif_xmit_frozen_or_stopped(txq))
+	    netif_xmit_frozen_or_stopped(txq)) {
+		clear_bit(__QDISC_STATE_MISSED, &q->state);
 		return skb;
+	}

The queues are woken asynchronously without holding any locks via
netif_tx_wake_queue(). Theoretically we can have a situation where:

CPU 0                            CPU 1   
  .                                .
dequeue_skb()                      .
  netif_xmit_frozen..() # true     .
  .                              [IRQ]
  .                              netif_tx_wake_queue()
  .                              <end of IRQ>
  .                              netif_tx_action()
  .                              set MISSED
  clear MISSED
  return NULL
ret from qdisc_restart()
ret from __qdisc_run()
qdisc_run_end()
-> MISSED not set

Yes, the above does seems to have the above data race.

As my understanding, there is two ways to fix the above data race:
1. do not clear the STATE_MISSED for netif_xmit_frozen_or_stopped()
   case, just check the netif_xmit_frozen_or_stopped() before
   calling __netif_schedule() at the end of qdisc_run_end(). This seems
   to only work with qdisc with TCQ_F_ONETXQUEUE flag because it seems
   we can only check the netif_xmit_frozen_or_stopped() with q->dev_queue,
   I am not sure q->dev_queue is pointint to which netdev queue when qdisc
   is not set with TCQ_F_ONETXQUEUE flag.

2. clearing the STATE_MISSED for netif_xmit_frozen_or_stopped() case
   as this patch does, and protect the __netif_schedule() with q->seqlock
   for netif_tx_wake_queue(), which might bring unnecessary overhead for
   non-stopped queue case

Any better idea?

.