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

On 2021/5/11 17:04, Yunsheng Lin wrote:

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

quoted

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?

3. Or check the netif_xmit_frozen_or_stopped() again after clearing
   STATE_MISSED, like below:

   if (netif_xmit_frozen_or_stopped(txq)) {
	  clear_bit(__QDISC_STATE_MISSED, &q->state);

	  /* Make sure the below netif_xmit_frozen_or_stopped()
	   * checking happens after clearing STATE_MISSED.
	   */
	  smp_mb__after_atomic();

	  /* Checking netif_xmit_frozen_or_stopped() again to
	   * make sure __QDISC_STATE_MISSED is set if the
	   * __QDISC_STATE_MISSED set by netif_tx_wake_queue()'s
	   * rescheduling of net_tx_action() is cleared by the
	   * above clear_bit().
	   */
	  if (!netif_xmit_frozen_or_stopped(txq))
	  	set_bit(__QDISC_STATE_MISSED, &q->state);
  }

  It is kind of ugly, but it does seem to fix the above data race too.
  And it seems like a common pattern to deal with the concurrency between
  xmit and NAPI polling, as below:

https://elixir.bootlin.com/linux/v5.12-rc2/source/drivers/net/ethernet/hisilicon/hns3/hns3_enet.c#L1409

quoted

.

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