[LEDE-DEV] Cake SQM killing my DIR-860L - was: [17.01] Kernel: bump to 4.4.51

[Martin Tippmann]

On Wed, Mar 1, 2017 at 11:40 PM, Weedy <weedy2887 at gmail.com> wrote:
> On 28 February 2017 at 05:40, Martin Tippmann <mt at i3o.de> wrote:
>> On Mon, Feb 27, 2017 at 9:17 PM, Stijn Segers
>> <francesco.borromini at inventati.org> wrote:
>>> Okay, so I tracked it down to cake being the culprit. When I disable the
>>> Cake SQM instance, no more of those traces, and no more sudden reboots.
>>>
>>> If I can help debug this, let me know - I enabled a Cake SQM instance on an
>>> APU2 and so far that seems to run fine.
>>
>> cake: Maybe it's related - I'm seeing high cpu usage with cake on
>> TP-Link 841N routers even with none, moderate traffic after a while. I
>> don't see hanging tasks in the logs but the system feels sluggish even
>> it's idle.
>
>
> I have a WR842ND v2, I'm also seeing high CPU suddenly with only
> default fq-codel on 4.4.50.

with fq_codel everything is fine for me (on WR841Nv7/8/11), even cake
is fine on a MAC1200R. We have a few a few WR842N in our community
mesh network but these are also remote and in use at the moment.

Just to be sure: Is the build newer than January 27? We've seen high
sys without these fixes:
https://git.lede-project.org/?p=source.git;a=commit;h=82d580e8b5c43f4dd228f2bb5927ca3e47752a34
https://git.lede-project.org/?p=source.git;a=commit;h=b94177e10fc72f9309eae7459c3570e5c080e960

> Are you able to git bisect? This particular device is installed in a
> remote location so I can't deal with it for a while.

Never done that but I guess it's possible. At the moment I'm not even
sure how to reliable reproduce the cake issue. Filling /tmp might
trigger it. I've discovered after routers got slow after uploading the
a new sysupgrade image to /tmp.

I wanted to play with trace-cmd (ftrace) after discovering that you
can use trace-cmd to record what's going on in the kernel remotly
using the -N option
(http://man7.org/linux/man-pages/man1/trace-cmd-record.1.html) but
this is pretty much all new to me.

The idea was to get a grasp where that high sys is coming from. Had no
time to play with this and I'm not sure if trace-cmd record is even
the right tool for this or works on my 32mb ram/4mb flash device.
http://www.brendangregg.com/linuxperf.html has a lot of stuff that
looks useful.

I don't have much time until the weekend, if I manage to get the
traces running I'll post them in the bugticket.

regards
Martin