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:

quoted

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 <redacted>
---

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/ (local)
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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