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

Yunsheng Lin
2021-05-31T12:40:19Z

On 2021/5/31 9:10, Yunsheng Lin wrote:
> 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?

I had did some interesting testing to show how adjust a small number
of code has some notiable performance degrade.

1. I used below patch to remove the nolock_qdisc_is_empty() testing
under q->seqlock.

@@ -3763,17 +3763,6 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
if (q->flags & TCQ_F_NOLOCK) {
if (q->flags & TCQ_F_CAN_BYPASS && nolock_qdisc_is_empty(q) &&
qdisc_run_begin(q)) {
- /* Retest nolock_qdisc_is_empty() within the protection
- * of q->seqlock to ensure qdisc is indeed empty.
- */
- if (unlikely(!nolock_qdisc_is_empty(q))) {
- rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
- __qdisc_run(q);
- qdisc_run_end(q);
-
- goto no_lock_out;
- }
-
qdisc_bstats_cpu_update(q, skb);
if (sch_direct_xmit(skb, q, dev, txq, NULL, true) &&
!nolock_qdisc_is_empty(q))
@@ -3786,7 +3775,6 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
qdisc_run(q);

-no_lock_out:
if (unlikely(to_free))
kfree_skb_list(to_free);
return rc;

which has the below performance improvement:

threads v1 v1 + above patch delta
1 3.21Mpps 3.20Mpps -0.3%
2 5.56Mpps 5.94Mpps +4.9%
4 5.58Mpps 5.60Mpps +0.3%
8 2.76Mpps 2.77Mpps +0.3%
16 2.23Mpps 2.23Mpps +0.0%

v1 = this patchset.

2. After the above testing, it seems worthwhile to remove the
nolock_qdisc_is_empty() testing under q->seqlock, so I used below
patch to make sure nolock_qdisc_is_empty() always return false for
netdev queue stopped case。

--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -38,6 +38,15 @@ EXPORT_SYMBOL(default_qdisc_ops);
static void qdisc_maybe_clear_missed(struct Qdisc *q,
const struct netdev_queue *txq)
{
+ set_bit(__QDISC_STATE_DRAINING, &q->state);
+
+ /* Make sure DRAINING is set before clearing MISSED
+ * to make sure nolock_qdisc_is_empty() always return
+ * false for aoviding transmitting a packet directly
+ * bypassing the requeued packet.
+ */
+ smp_mb__after_atomic();
+
clear_bit(__QDISC_STATE_MISSED, &q->state);

/* Make sure the below netif_xmit_frozen_or_stopped()
@@ -52,8 +61,6 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q,
*/
if (!netif_xmit_frozen_or_stopped(txq))
set_bit(__QDISC_STATE_MISSED, &q->state);
- else
- set_bit(__QDISC_STATE_DRAINING, &q->state);
}

which has the below performance data:

threads v1 v1 + above two patch delta
1 3.21Mpps 3.20Mpps -0.3%
2 5.56Mpps 5.94Mpps +4.9%
4 5.58Mpps 5.02Mpps -10%
8 2.76Mpps 2.77Mpps +0.3%
16 2.23Mpps 2.23Mpps +0.0%

So the adjustment in qdisc_maybe_clear_missed() seems to have
caused about 10% performance degradation for 4 threads case.

And the cpu topdown perf data suggested that icache missed and
bad Speculation play the main factor to those performance difference.

I tried to control the above factor by removing the inline function
and add likely and unlikely tag for netif_xmit_frozen_or_stopped()
in sch_generic.c.

And after removing the inline mark for function in sch_generic.c
and add likely/unlikely tag for netif_xmit_frozen_or_stopped()
checking in in sch_generic.c, we got notiable performance improvement
for 1/2 threads case(some performance improvement for ip forwarding
test too), but not for 4 threads case.

So it seems we need to ignore the performance degradation for 4
threads case? or any idea?

>>>
>>> Sorry for dragging the review out..
>>>
>>> .
>>>
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