netdev - Re: 4.6.3, pppoe + shaper workload, skb_panic / skb_push / ppp_start

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Date:	Tue, 12 Jul 2016 21:13:11 +0300
From:	nuclearcat@...learcat.com
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: 4.6.3, pppoe + shaper workload, skb_panic / skb_push /
 ppp_start_xmit

On 2016-07-12 21:05, Cong Wang wrote:
> On Tue, Jul 12, 2016 at 11:03 AM,  <nuclearcat@...learcat.com> wrote:
>> On 2016-07-12 20:31, Cong Wang wrote:
>>> 
>>> On Mon, Jul 11, 2016 at 12:45 PM,  <nuclearcat@...learcat.com> wrote:
>>>> 
>>>> Hi
>>>> 
>>>> On latest kernel i noticed kernel panic happening 1-2 times per day. 
>>>> It
>>>> is
>>>> also happening on older kernel (at least 4.5.3).
>>>> 
>>> ...
>>>> 
>>>>  [42916.426463] Call Trace:
>>>>  [42916.426658]  <IRQ>
>>>> 
>>>>  [42916.426719]  [<ffffffff81843786>] skb_push+0x36/0x37
>>>>  [42916.427111]  [<ffffffffa00e8ce5>] ppp_start_xmit+0x10f/0x150
>>>> [ppp_generic]
>>>>  [42916.427314]  [<ffffffff81853467>] 
>>>> dev_hard_start_xmit+0x25a/0x2d3
>>>>  [42916.427516]  [<ffffffff818530f2>] ?
>>>> validate_xmit_skb.isra.107.part.108+0x11d/0x238
>>>>  [42916.427858]  [<ffffffff8186dee3>] sch_direct_xmit+0x89/0x1b5
>>>>  [42916.428060]  [<ffffffff8186e142>] __qdisc_run+0x133/0x170
>>>>  [42916.428261]  [<ffffffff81850034>] net_tx_action+0xe3/0x148
>>>>  [42916.428462]  [<ffffffff810c401a>] __do_softirq+0xb9/0x1a9
>>>>  [42916.428663]  [<ffffffff810c4251>] irq_exit+0x37/0x7c
>>>>  [42916.428862]  [<ffffffff8102b8f7>] 
>>>> smp_apic_timer_interrupt+0x3d/0x48
>>>>  [42916.429063]  [<ffffffff818cb15c>] apic_timer_interrupt+0x7c/0x90
>>> 
>>> 
>>> Interesting, we call a skb_cow_head() before skb_push() in
>>> ppp_start_xmit(),
>>> I have no idea why this could happen.
>>> 
>>> Do you have any tc qdisc, filter or actions on this ppp device?
>> 
>> Yes, i have policing filters for incoming traffic (ingress), and also 
>> on
>> egress htb + pfifo + filters.
> 
> Does it make any difference if you remove the egress qdisc and/or
> filters? If yes, please share the `tc qd show...` and `tc filter show 
> ...`?
> 
> Thanks!

It is not easy, because it is NAS with approx 5000 users connected (and 
they are constantly connecting/disconnecting), and crash can't be 
reproduced easily. If i will remove qdisc/filters users will get 
unlimited speed and this will cause serious service degradation.
But maybe i can add some debug lines and run some test kernel if 
necessary (if it will not cause serious performance overhead).