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,  [off-list ref] wrote:

quoted

On 2016-07-12 20:31, Cong Wang wrote:

quoted

On Mon, Jul 11, 2016 at 12:45 PM,  [off-list ref] wrote:

quoted

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).

...

quoted

 [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).