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@huawei.com>
I gave it a short test on a single core imx. No problem here. Marc -- Pengutronix e.K. | Marc Kleine-Budde | Embedded Linux | https://www.pengutronix.de | Vertretung West/Dortmund | Phone: +49-231-2826-924 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |