Messages in this thread |  | | | Date | Thu, 02 Oct 2008 17:57:25 -0600 | | From | "Chris Friesen" <> | | Subject | [bug report] sched: stop_machine() usage causes load balancer to misbehave |
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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
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