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

[Jakub Kicinski <kuba@kernel.org>]

On Sat, 29 May 2021 09:44:57 +0800 Yunsheng Lin wrote:
> >> @@ -3852,10 +3852,32 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
> >>  	qdisc_calculate_pkt_len(skb, 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))) {  
> > 
> > This is just for the DRAINING case right? 
> > 
> > MISSED can be set at any moment, testing MISSED seems confusing.  
> 
> MISSED is only set when there is lock contention, which means it
> is better not to do the qdisc bypass to avoid out of order packet
> problem, 

Avoid as in make less likely? Nothing guarantees other thread is not
interrupted after ->enqueue and before qdisc_run_begin().

TBH I'm not sure what out-of-order situation you're referring to,
there is no ordering guarantee between separate threads trying to
transmit AFAIU.

IOW this check is not required for correctness, right?

> another good thing is that we could also do the batch
> dequeuing and transmiting of packets when there is lock contention.

No doubt, but did you see the flag get set significantly often here 
to warrant the double-checking?

> > Is it really worth the extra code?  
> 
> Currently DRAINING is only set for the netdev queue stopped.
> We could only use DRAINING to indicate the non-empty of a qdisc,
> then we need to set the DRAINING evrywhere MISSED is set, that is
> why I use both DRAINING and MISSED to indicate a non-empty qdisc.
> 
> >   
> >> +				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))
> >> +				__qdisc_run(q);
> >> +
> >> +			qdisc_run_end(q);
> >> +			return NET_XMIT_SUCCESS;
> >> +		}
> >> +
> >>  		rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
> >> -		if (likely(!netif_xmit_frozen_or_stopped(txq)))
> >> -			qdisc_run(q);
> >> +		qdisc_run(q);
> >>  
> >> +no_lock_out:
> >>  		if (unlikely(to_free))
> >>  			kfree_skb_list(to_free);
> >>  		return rc;

> >> @@ -164,9 +166,13 @@ static inline void dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
> >>  
> >>  		skb = next;
> >>  	}
> >> -	if (lock)
> >> +
> >> +	if (lock) {
> >>  		spin_unlock(lock);
> >> -	__netif_schedule(q);
> >> +		set_bit(__QDISC_STATE_MISSED, &q->state);
> >> +	} else {
> >> +		__netif_schedule(q);  
> > 
> > Could we reorder qdisc_run_begin() with clear_bit(SCHED) 
> > in net_tx_action() and add SCHED to the NON_EMPTY mask?  
> 
> Did you mean clearing the SCHED after the q->seqlock is
> taken?
> 
> The problem is that the SCHED is also used to indicate
> a qdisc is in sd->output_queue or not, and the
> qdisc_run_begin() called by net_tx_action() can not
> guarantee it will take the q->seqlock(we are using trylock
> for lockless qdisc)

Ah, right. We'd need to do some more flag juggling in net_tx_action()
to get it right.

> >> +	}
> >>  }
> >>  
> >>  static void try_bulk_dequeue_skb(struct Qdisc *q,
> >> @@ -409,7 +415,11 @@ void __qdisc_run(struct Qdisc *q)
> >>  	while (qdisc_restart(q, &packets)) {
> >>  		quota -= packets;
> >>  		if (quota <= 0) {
> >> -			__netif_schedule(q);
> >> +			if (q->flags & TCQ_F_NOLOCK)
> >> +				set_bit(__QDISC_STATE_MISSED, &q->state);
> >> +			else
> >> +				__netif_schedule(q);
> >> +
> >>  			break;
> >>  		}
> >>  	}
> >> @@ -680,13 +690,14 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
> >>  	if (likely(skb)) {
> >>  		qdisc_update_stats_at_dequeue(qdisc, skb);
> >>  	} else if (need_retry &&
> >> -		   test_bit(__QDISC_STATE_MISSED, &qdisc->state)) {
> >> +		   READ_ONCE(qdisc->state) & QDISC_STATE_NON_EMPTY) {  
> > 
> > Do we really need to retry based on DRAINING being set?
> > Or is it just a convenient way of coding things up?  
> 
> Yes, it is just a convenient way of coding things up.
> Only MISSED need retrying.