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

[Jakub Kicinski]

On Mon, 10 May 2021 09:42:36 +0800 Yunsheng Lin wrote:
> 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 <mkubecek@xxxxxxx>
> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>

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

> @@ -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