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@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
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?
>
> .
>
