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DateThu, 02 Oct 2008 17:57:25 -0600
From"Chris Friesen" <>
Subject[bug report] sched: stop_machine() usage causes load balancer to misbehave
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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