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

[Yunsheng Lin <linyunsheng@huawei.com>]

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

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


> 
>>
>> .
>>
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