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

Jakub Kicinski
2021-06-01T04:51:59Z

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:
> [...]
> [...]
> [...]
> [...]
> [...]
> [...]
> [...]
> >>
> >> 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.

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.
+ */

> 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()?

> 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?