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

[Marc Kleine-Budde]

Cc += linux-can@xxxxxxxxxxxxxxx

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

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

Marc

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