Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

[Marc Kleine-Budde]

On 3/15/21 4:10 AM, Yunsheng Lin wrote:
> Currently pfifo_fast has both TCQ_F_CAN_BYPASS and TCQ_F_NOLOCK
> flag set, but queue discipline by-pass does not work for lockless
> qdisc because skb is always enqueued to qdisc even when the qdisc
> is empty, see __dev_xmit_skb().
> 
> This patch calls sch_direct_xmit() to transmit the skb directly
> to the driver for empty lockless qdisc too, which aviod enqueuing
> and dequeuing operation. qdisc->empty is set to false whenever a
> skb is enqueued, see pfifo_fast_enqueue(), and is set to true when
> skb dequeuing return NULL, see pfifo_fast_dequeue(), a spinlock is
> added to avoid the race between enqueue/dequeue and qdisc->empty
> setting.
> 
> If there is requeued skb in q->gso_skb, and qdisc->empty is true,
> do not allow bypassing requeued skb. enqueuing and dequeuing in
> q->gso_skb is always protected by qdisc->seqlock, so is the access
> of q->gso_skb by skb_queue_empty();
> 
> Also, qdisc is scheduled at the end of qdisc_run_end() when q->empty
> is false to avoid packet stuck problem.
> 
> The performance for ip_forward test increases about 10% with this
> patch.
> 
> Signed-off-by: Yunsheng Lin <linyunsheng@xxxxxxxxxx>

I gave it a short test on a single core imx. No problem here.

Marc

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