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@xxxxxxxxxx> > --- > 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 CPU2 > qdisc_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.
