Re: [RFC v3] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

[Vladimir Oltean]

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.