# Re: \[tip:sched/core\] sched/core: Add debugging code to catch missing update\_rq\_clock() calls

- From: Sachin Sant \<hidden\>
- Date: 2017-02-06 06:23:31 
- List: linuxppc-dev
- Page: https://ratatoskr.run/linuxppc-dev/2017/02/4927735
- Thread: https://ratatoskr.run/linuxppc-dev/2017/01/8900177/t.md

```text

>>> I've seen it on tip. It looks like hot unplug goes really slow when
>>> there's running tasks on the CPU being taken down.
>>>=20
>>> What I did was something like:
>>>=20
>>>  taskset -p $((1<<1)) $$
>>>  for ((i=3D0; i<20; i++)) do while :; do :; done & done
>>>=20
>>>  taskset -p $((1<<0)) $$
>>>  echo 0 > /sys/devices/system/cpu/cpu1/online
>>>=20
>>> And with those 20 tasks stuck sucking cycles on CPU1, the unplug =
goes
>>> _really_ slow and the RCU stall triggers. What I suspect happens is =
that
>>> hotplug stops participating in the RCU state machine early, but only
>>> tells RCU about it really late, and in between it gets suspicious it
>>> takes too long.
>>>=20
>>> I've yet to dig through the RCU code to figure out the exact =
sequence of
>>> events, but found the above to be fairly reliable in triggering the
>>> issue.
>=20
>> If you send me the full splat from the dmesg and the RCU portions of
>> .config, I will take a look.  Is this new behavior, or a new test?
>=20

I have sent the required files to you via separate email.

> If new behavior, I would be most suspicious of these commits in -rcu =
which
> recently entered -tip:
>=20
> 19e4d983cda1 rcu: Place guard on rcu_all_qs() and =
rcu_note_context_switch() actions
> 913324b1364f rcu: Eliminate flavor scan in =
rcu_momentary_dyntick_idle()
> fcdcfefafa45 rcu: Pull rcu_qs_ctr into rcu_dynticks structure
> 0919a0b7e7a5 rcu: Pull rcu_sched_qs_mask into rcu_dynticks structure
> caa7c8e34293 rcu: Make rcu_note_context_switch() do deferred NOCB =
wakeups
> 41e4b159d516 rcu: Make rcu_all_qs() do deferred NOCB wakeups
> b457a3356a68 rcu: Make call_rcu() do deferred NOCB wakeups
>=20
> Does reverting any of these help?

I tried reverting the above commits. That does not help. I can still =
recreate the issue.

Thanks
-Sachin=
```