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

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2021/3/14 18:15, Marc Kleine-Budde wrote:
> Cc += linux-can@vger.kernel.org
> 
> On 3/14/21 1:03 AM, Vladimir Oltean wrote:
>> On Sat, Mar 13, 2021 at 10:47:47AM +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 calles 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, and is set to true when skb dequeuing return NULL,
>>> see pfifo_fast_dequeue().
>>>
>>> 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 can confirm the ~10% IP forwarding throughput improvement brought by
>> this patch, but as you might be aware, there was a previous attempt to
>> add qdisc bypass to pfifo_fast by Paolo Abeni:
>> https://lore.kernel.org/netdev/661cc33a-5f65-2769-cc1a-65791cb4b131@pengutronix.de/

Thanks for mention the previous attempt to add qdisc bypass to pfifo_fast.

>> It was reverted because TX reordering was observed with SocketCAN
>> (although, presumably it should also be seen with Ethernet and such).

When writing this patch, I was more foucusing on packet stuck problem
when TCQ_F_CAN_BYPASS is added for lockless qdisc.

When I am looking at flexcan_start_xmit() used by the can driver you mentioned,
it calls netif_stop_queue() to disable the queue when sending each skb, which may
cuause other skb to be requeued, see dev_requeue_skb() called by sch_direct_xmit(),
and q->empty is still true when this happens, so other cpu may send skb directly
bypassing the requeued skb, causing an out of order problem.

I will try to deal with the above requeued skb problem, and see if there are other
timing issus beside the requeued skb problem.

Thanks for the testing again.

> 
> Thanks for testing that, I just stumbled over this patch by accident.
> 
> Marc
>