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.
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
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()