Re: [Linuxarm] Re: [PATCH net-next 2/3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/5/31 8:40, Yunsheng Lin wrote:
> On 2021/5/31 4:21, Jakub Kicinski wrote:
>> On Sun, 30 May 2021 09:37:09 +0800 Yunsheng Lin wrote:
>>> On 2021/5/30 2:49, Jakub Kicinski wrote:
>>>> The fact that MISSED is only cleared under q->seqlock does not matter,
>>>> because setting it and ->enqueue() are not under any lock. If the thread
>>>> gets interrupted between:
>>>>
>>>> 	if (q->flags & TCQ_F_CAN_BYPASS && nolock_qdisc_is_empty(q) &&
>>>> 	    qdisc_run_begin(q)) {
>>>>
>>>> and ->enqueue() we can't guarantee that something else won't come in,
>>>> take q->seqlock and clear MISSED.
>>>>
>>>> thread1                thread2             thread3
>>>> # holds seqlock
>>>>                        qdisc_run_begin(q)
>>>>                        set(MISSED)
>>>> pfifo_fast_dequeue
>>>>   clear(MISSED)
>>>>   # recheck the queue
>>>> qdisc_run_end()  
>>>>                        ->enqueue()  
>>>>                                             q->flags & TCQ_F_CAN_BYPASS..
>>>>                                             qdisc_run_begin() # true
>>>>                                             sch_direct_xmit()
>>>>                        qdisc_run_begin()
>>>>                        set(MISSED)
>>>>
>>>> Or am I missing something?
>>>>
>>>> Re-checking nolock_qdisc_is_empty() may or may not help.
>>>> But it doesn't really matter because there is no ordering
>>>> requirement between thread2 and thread3 here.  
>>>
>>> I were more focued on explaining that using MISSED is reliable
>>> as sch_may_need_requeuing() checking in RFCv3 [1] to indicate a
>>> empty qdisc, and forgot to mention the data race described in
>>> RFCv3, which is kind of like the one described above:
>>>
>>> "There is a data race as below:
>>>
>>>       CPU1                                   CPU2
>>> qdisc_run_begin(q)                            .
>>>         .                                q->enqueue()
>>> sch_may_need_requeuing()                      .
>>>     return true                               .
>>>         .                                     .
>>>         .                                     .
>>>     q->enqueue()                              .
>>>
>>> When above happen, the skb enqueued by CPU1 is dequeued after the
>>> skb enqueued by CPU2 because sch_may_need_requeuing() return true.
>>> If there is not qdisc bypass, the CPU1 has better chance to queue
>>> the skb quicker than CPU2.
>>>
>>> This patch does not take care of the above data race, because I
>>> view this as similar as below:
>>>
>>> Even at the same time CPU1 and CPU2 write the skb to two socket
>>> which both heading to the same qdisc, there is no guarantee that
>>> which skb will hit the qdisc first, becuase there is a lot of
>>> factor like interrupt/softirq/cache miss/scheduling afffecting
>>> that."
>>>
>>> Does above make sense? Or any idea to avoid it?
>>>
>>> 1. https://patchwork.kernel.org/project/netdevbpf/patch/1616404156-11772-1-git-send-email-linyunsheng@huawei.com/
>>
>> We agree on this one.
>>
>> Could you draw a sequence diagram of different CPUs (like the one
>> above) for the case where removing re-checking nolock_qdisc_is_empty()
>> under q->seqlock leads to incorrect behavior? 
> 
> When nolock_qdisc_is_empty() is not re-checking under q->seqlock, we
> may have:
> 
> 
>         CPU1                                   CPU2
>   qdisc_run_begin(q)                            .
>           .                                enqueue skb1
> deuqueue skb1 and clear MISSED                  .
>           .                        nolock_qdisc_is_empty() return true
>     requeue skb                                 .
>    q->enqueue()                                 .
>     set MISSED                                  .
>         .                                       .
>  qdisc_run_end(q)                               .
>         .                              qdisc_run_begin(q)
>         .                             transmit skb2 directly
>         .                           transmit the requeued skb1
> 
> The problem here is that skb1 and skb2  are from the same CPU, which
> means they are likely from the same flow, so we need to avoid this,
> right?


         CPU1                                   CPU2
   qdisc_run_begin(q)                            .
           .                                enqueue skb1
     dequeue skb1                                .
           .                                     .
netdevice stopped and MISSED is clear            .
           .                        nolock_qdisc_is_empty() return true
     requeue skb                                 .
           .                                     .
           .                                     .
           .                                     .
  qdisc_run_end(q)                               .
           .                              qdisc_run_begin(q)
           .                             transmit skb2 directly
           .                           transmit the requeued skb1

The above sequence diagram seems more correct, it is basically about how to
avoid transmitting a packet directly bypassing the requeued packet.

> 
>>
>> If there is no such case would you be willing to repeat the benchmark
>> with and without this test?
>>
>> Sorry for dragging the review out..
>>
>> .
>>
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