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

Yunsheng Lin
2021-06-01T08:18:59Z

On 2021/6/1 12:51, Jakub Kicinski wrote:
> On Mon, 31 May 2021 20:40:01 +0800 Yunsheng Lin wrote:
>> 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:
>> [...] >>>
>>>
>>> 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.
>
> I see, thanks! That explains the need. Perhaps we can rephrase the
> comment? Maybe:
>
> + /* Retest nolock_qdisc_is_empty() within the protection
> + * of q->seqlock to protect from racing with requeuing.
> + */

Yes if we still decide to preserve the nolock_qdisc_is_empty() rechecking
under q->seqlock.

>
>> 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);
>> }
>
> But this would not be enough because we may also clear MISSING
> in pfifo_fast_dequeue()?

For the MISSING clearing in pfifo_fast_dequeue(), it seems it
looks like the data race described in RFC v3 too?

CPU1 CPU2 CPU3
qdisc_run_begin(q) . .
. MISSED is set .
MISSED is cleared . .
q->dequeue() . .
. enqueue skb1 check MISSED # true
qdisc_run_end(q) . .
. . qdisc_run_begin(q) # true
. MISSED is set send skb2 directly

>
>> 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?
>
> No ideas, are the threads pinned to CPUs in some particular way?

The pktgen seems already runnig a thread for each CPU, so I do not
need to do the pinning myself, for the 4 threads case, it runs on
the 0~3 cpu.

It seems more related to specific cpu implemantaion.

>
> .
>