From: Vladimir Oltean <olteanv@gmail.com>
To: Yunsheng Lin <linyunsheng@huawei.com>
Cc: davem@davemloft.net, kuba@kernel.org, ast@kernel.org,
daniel@iogearbox.net, andriin@fb.com, edumazet@google.com,
weiwan@google.com, cong.wang@bytedance.com, ap420073@gmail.com,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
linuxarm@openeuler.org, pabeni@redhat.com
Subject: Re: [PATCH RFC] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc
Date: Sun, 14 Mar 2021 02:03:50 +0200 [thread overview]
Message-ID: <20210314000350.2mrhvprsi77qwqdi@skbuf> (raw)
In-Reply-To: <1615603667-22568-1-git-send-email-linyunsheng@huawei.com>
Hi Yunsheng,
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/
It was reverted because TX reordering was observed with SocketCAN
(although, presumably it should also be seen with Ethernet and such).
In fact I have a setup with two NXP LS1028A-RDB boards (which use the
drivers/net/can/flexcan.c driver and the pfifo_fast qdisc):
+-----------+ +-----------+
| | | |
| Generator | | DUT |
| |--------------------->| |
| canfdtest | reflector | canfdtest |
| |<---------------------| |
| can1 | | can0 |
| | | |
+-----------+ +-----------+
where reordering happens in the TX side of the DUT and is noticed in the
RX side of the generator. The test frames are classic CAN, not CAN FD.
I was able to run the canfdtest described above successfully for several
hours (10 million CAN frames) on the current net-next (HEAD at commit
34bb97512641 ("net: fddi: skfp: Mundane typo fixes throughout the file
smt.h")) with no reordering.
Then, after applying your patch, I am seeing TX reordering within a few
minutes (less than 100K frames sent), therefore this reintroduces the
bug due to which Paolo's patch was reverted.
Sadly I am not knowledgeable enough to give you any hints as to what is
going wrong, but in case you have any ideas for debug, I would be glad
to test them out on my boards.
next prev parent reply other threads:[~2021-03-14 0:05 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-13 2:47 [PATCH RFC] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc Yunsheng Lin
2021-03-14 0:03 ` Vladimir Oltean [this message]
2021-03-14 10:15 ` Marc Kleine-Budde
2021-03-15 0:50 ` Yunsheng Lin
2021-03-15 3:10 ` [RFC v2] " Yunsheng Lin
2021-03-15 12:29 ` Vladimir Oltean
2021-03-15 13:09 ` Marc Kleine-Budde
2021-03-15 18:53 ` Jakub Kicinski
2021-03-16 0:35 ` Yunsheng Lin
2021-03-16 3:47 ` [Linuxarm] " Yunsheng Lin
2021-03-16 8:15 ` Eric Dumazet
2021-03-16 12:36 ` Yunsheng Lin
2021-03-16 22:48 ` Cong Wang
2021-03-17 1:14 ` Yunsheng Lin
2021-03-17 13:35 ` Toke Høiland-Jørgensen
2021-03-17 13:45 ` Jason A. Donenfeld
2021-03-18 7:33 ` [Linuxarm] " Yunsheng Lin
2021-03-19 18:15 ` Cong Wang
2021-03-22 0:55 ` Yunsheng Lin
2021-03-24 1:49 ` Cong Wang
2021-03-24 2:36 ` Yunsheng Lin
2021-03-19 19:03 ` Jason A. Donenfeld
2021-03-22 1:05 ` Yunsheng Lin
2021-03-18 7:10 ` Ahmad Fatoum
2021-03-18 7:46 ` Yunsheng Lin
2021-03-18 9:09 ` Ahmad Fatoum
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