On 2021/5/11 12:22, Jakub Kicinski wrote:
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@suse.cz> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
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
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?
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