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
