linux-kernel - Re: [bug, bisected] pfifo_fast causes packet reordering

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Message-ID: <CAA93jw5FSbvg8AZ=EmneW8juqd8LCgp=xUrSxYm2GBEucKkT3w@mail.gmail.com>
Date:   Tue, 13 Mar 2018 11:35:46 -0700
From:   Dave Taht <dave.taht@...il.com>
To:     Jakob Unterwurzacher <jakob.unterwurzacher@...obroma-systems.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        John Fastabend <john.fastabend@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        Martin Elshuber <martin.elshuber@...obroma-systems.com>
Subject: Re: [bug, bisected] pfifo_fast causes packet reordering

On Tue, Mar 13, 2018 at 11:24 AM, Jakob Unterwurzacher
<jakob.unterwurzacher@...obroma-systems.com> wrote:
> During stress-testing our "ucan" USB/CAN adapter SocketCAN driver on Linux
> v4.16-rc4-383-ged58d66f60b3 we observed that a small fraction of packets are
> delivered out-of-order.
>
> We have tracked the problem down to the driver interface level, and it seems
> that the driver's net_device_ops.ndo_start_xmit() function gets the packets
> handed over in the wrong order.
>
> This behavior was not observed on Linux v4.15 and I have bisected the
> problem down to this patch:
>
>> commit c5ad119fb6c09b0297446be05bd66602fa564758
>> Author: John Fastabend <john.fastabend@...il.com>
>> Date:   Thu Dec 7 09:58:19 2017 -0800
>>
>>    net: sched: pfifo_fast use skb_array
>>
>>    This converts the pfifo_fast qdisc to use the skb_array data structure
>>    and set the lockless qdisc bit. pfifo_fast is the first qdisc to
>> support
>>    the lockless bit that can be a child of a qdisc requiring locking. So
>>    we add logic to clear the lock bit on initialization in these cases
>> when
>>    the qdisc graft operation occurs.
>>
>>    This also removes the logic used to pick the next band to dequeue from
>>    and instead just checks a per priority array for packets from top
>> priority
>>    to lowest. This might need to be a bit more clever but seems to work
>>    for now.
>>
>>    Signed-off-by: John Fastabend <john.fastabend@...il.com>
>>    Signed-off-by: David S. Miller <davem@...emloft.net>
>
>
> The patch does not revert cleanly, but moving to one commit earlier makes
> the problem go away.
>
> Selecting the "fq" scheduler instead of "pfifo_fast" makes the problem go
> away as well.

I am of course, a fan of obsoleting pfifo_fast. There's no good reason
for it anymore.

>
> Is this an unintended side-effect of the patch or is there something the
> driver has to do to request in-order delivery?
>
> Thanks,
> Jakob



-- 

Dave Täht
CEO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-669-226-2619