[tip:sched/core] sched/core: Add debugging code to catch missing update_rq_clock() calls

[Sachin Sant]

>>> 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.
>>> 
>>> What I did was something like:
>>> 
>>>  taskset -p $((1<<1)) $$
>>>  for ((i=0; i<20; i++)) do while :; do :; done & done
>>> 
>>>  taskset -p $((1<<0)) $$
>>>  echo 0 > /sys/devices/system/cpu/cpu1/online
>>> 
>>> 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.
>>> 
>>> 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.
> 
>> 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?
> 

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:
> 
> 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
> 
> Does reverting any of these help?

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

Thanks
-Sachin