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

[Yunsheng Lin]

On 2021/5/12 7:30, Jakub Kicinski wrote:
> On Tue, 11 May 2021 20:13:56 +0800 Yunsheng Lin wrote:
>> On 2021/5/11 17:04, Yunsheng Lin wrote:
>>> On 2021/5/11 12:22, Jakub Kicinski wrote:  
>>>> 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()  
>>  [...]  
>>>
>>> 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.
> 
> Isn't the case where we have a NOLOCK qdisc without TCQ_F_ONETXQUEUE
> rather unexpected? It'd have to be a single pfifo on multi-queue
> netdev, right? Sounds not worth optimizing for. How about:
> 
>  static inline void qdisc_run_end(struct Qdisc *qdisc)
>  {
>  	write_seqcount_end(&qdisc->running);
> 	if (qdisc->flags & TCQ_F_NOLOCK) {
>  		spin_unlock(&qdisc->seqlock);
> 
> 		if (unlikely(test_bit(__QDISC_STATE_MISSED,
> 				      &qdisc->state))) {
> 			clear_bit(__QDISC_STATE_MISSED, &qdisc->state);
> 			if (!(q->flags & TCQ_F_ONETXQUEUE) ||
> 			    !netif_xmit_frozen_or_stopped(q->dev_queue))
> 				__netif_schedule(qdisc);
> 		}
> 	}
>  }
> 
> For the strange non-ONETXQUEUE case we'd have an occasional unnecessary
> net_tx_action, but no infinite loop possible.
> 
>>> 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
> 
> This is indeed the idiomatic way of dealing with Tx queue stopping race,
> but it's a bit of code to sprinkle around. My vote would be option 1.

I had done some performance testing to see which is better, tested using
pktgen and dummy netdev with pfifo_fast qdisc attached:

unit: Mpps

threads    V6         V6 + option 1     V6 + option 3
  1       2.60          2.54               2.60
  2       3.86          3.84               3.84
  4       5.56          5.50               5.51
  8       2.79          2.77               2.77
  16      2.23          2.24               2.22

So it seems the netif_xmit_frozen_or_stopped checking overhead for non-stopped queue
is noticable for 1 pktgen thread.

And the performance increase for V6 + option 1 with 16 pktgen threads is because of
"clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at the end of qdisc_run_end(), which
may avoid the another round of dequeuing in the pfifo_fast_dequeue(). And adding the
"clear_bit(__QDISC_STATE_MISSED, &qdisc->state)"  for V6 + option 3, the data for
16 pktgen thread also go up to 2.24Mpps.


So it seems V6 + option 3 with "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at
the end of qdisc_run_end() is better?


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