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

Yunsheng Lin
2021-05-31T01:10:29Z

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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