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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/6/1 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.

> 
> .
>