[bug report] sched: stop_machine() usage causes load balancer to misbehave

["Chris Friesen" <cfriesen@nortel.com>]

I mentioned before that ftrace (specifically the ftraced daemon) seems 
to be interfering with the load balancer.  After some experimenting, it 
appears that any regular calls to stop_machine() will end up confusing 
the load balancer.

As an experiment, I disabled ftraced (which would normally result in 
correct load balancing) but added a single kernel thread which simply 
runs the following loop, where "chrisd2" is a dummy function.

while(1) {
	set_current_state(TASK_INTERRUPTIBLE);
	schedule_timeout(HZ);
	stop_machine(chrisd2, NULL, NULL);
}

With the modified kernel, my testcase shows that the load balancer 
doesn't balance--all tasks remain on one cpu while the other one stays idle.

Most of the users of stop_machine() (kprobes on s390, cpu hotplug, 
module load/unload, numa_zonelist_order, etc.) don't seem to be called 
on a regular basis.  Only ftrace behaves this way, which is why it 
appeared to be the source of the problem.

I haven't tracked down the specific reasons for the misbehaviour, but it 
seems undesirable.

Anyone have any ideas what might be causing this?  Is it a problem with 
the load balancer, or an unavoidable consequence of what stop_machine() 
is doing?

Thanks,

Chris