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 CPU2qdisc_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?
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?
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..