Hi Yunsheng,
On Mon, Mar 22, 2021 at 05:09:16PM +0800, 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().
There is a data race between enqueue/dequeue and qdisc->empty setting, qdisc->empty is only used as a hint, so we need to call sch_may_need_requeuing() to see if the queue is really empty and if there is requeued skb, which has higher priority than the current skb.
The performance for ip_forward test increases about 10% with this patch.
Signed-off-by: Yunsheng Lin linyunsheng@huawei.com
Hi, Vladimir and Ahmad Please give it a test to see if there is any out of order packet for this patch, which has removed the priv->lock added in RFC v2.
There is a data race as below:
CPU1 CPU2qdisc_run_begin(q) . . q->enqueue() sch_may_need_requeuing() . return true . . . . . q->enqueue() .
When above happen, the skb enqueued by CPU1 is dequeued after the skb enqueued by CPU2 because sch_may_need_requeuing() return true. If there is not qdisc bypass, the CPU1 has better chance to queue the skb quicker than CPU2.
This patch does not take care of the above data race, because I view this as similar as below:
Even at the same time CPU1 and CPU2 write the skb to two socket which both heading to the same qdisc, there is no guarantee that which skb will hit the qdisc first, becuase there is a lot of factor like interrupt/softirq/cache miss/scheduling afffecting that.
So I hope the above data race will not cause problem for Vladimir and Ahmad.
Preliminary results on my test setup look fine, but please allow me to run the canfdtest overnight, since as you say, races are still theoretically possible.