[ClusterLabs] corosync SCHED_RR stuck at 100% cpu usage with kernel 4.19, priority inversion/livelock?

[Edwin Török]

On 15/02/2019 16:08, Christine Caulfield wrote:
> On 15/02/2019 13:06, Edwin Török wrote:
>> I tried again with 'debug: trace', lots of process pause here:
>> https://clbin.com/ZUHpd
>>
>> And here is an strace running realtime prio 99, a LOT of epoll_wait and
>> sendmsg (gz format):
>> https://clbin.com/JINiV
>>
>> It detects large numbers of members left, but I think this is because
>> the corosync on those hosts got similarly stuck:
>> Feb 15 12:51:07 localhost corosync[29278]:  [TOTEM ] A new membership
>> (10.62.161.158:3152) was formed. Members left: 2 14 3 9 5 11 4 12 8 13 7
>> 1 10
>> Feb 15 12:51:07 localhost corosync[29278]:  [TOTEM ] Failed to receive
>> the leave message. failed: 2 14 3 9 5 11 4 12 8 13 7 1 10
>>
>> Looking on another host that is still stuck 100% corosync it says:
>> https://clbin.com/6UOn6
>>
> 
> Thanks, that's really quite odd. I have vague recollections of a problem
> where corosync was spinning on epoll without reading anything but can't
> find the details at the moment, annoying.
> 
> Some thing you might be able to try that might help.
> 
> 1) is is possible to run without sbd. Sometimes too much polling from
> clients can cause odd behaviour
> 2) is it possible to try with a different kernel? We've tried a vanilla
> 4.19 and it's fine, but not with the Xen patches obviously

I'll try with some bare-metal upstream distros and report back the repro
steps if I can get it to reliably repro, hopefully early next week, it
is unlikely I'll get a working repro today.

> 3) Does running corosync with the -p option help?

Yes, with "-p" I was able to run cluster create/GFS2 plug/unplug/destroy
on 16 physical hosts in a loop for an hour with any crashes (previously
it would crash within minutes).

I found another workaround too:
echo NO_RT_RUNTIME_SHARE >/sys/kernel/debug/sched_features

This makes the 95% realtime process CPU limit from
sched_rt_runtime_us/sched_rt_period_us apply per core, instead of
globally, so there would be 5% time left for non-realtime tasks on each
core. Seems to be enough to avoid the livelock, I was not able to
observe corosync using high CPU % anymore.
Still got more tests to run on this over the weekend, but looks promising.

This is a safety layer of course, to prevent the system from fencing if
we encounter high CPU usage in corosync/libq. I am still interested in
tracking down the corosync/libq issue as it shouldn't have happened in
the first place.

> 
> Is there any situation where this has worked? either with different
> components or different corosync.conf files?
> 
> Also, and I don't think this is directly related to the issue, but I can
> see configuration reloads happening from 2 nodes every 5 seconds. It's
> very odd and maybe not what you want!

The configuration reloads are a way of triggering this bug reliably, I
should've mentioned that earlier
(the problem happens during a configuration reload, but not always, and
by doing configuration reloads in a loop that just add/remove one node
the problem can be triggered reliably within minutes).


Best regards,
--Edwin