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SubjectRe: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes
Hi Satoru,

On Mon, May 07, 2007 at 07:10:05PM +0900, Satoru Takeuchi wrote:
> Hi,
> I found a bug on 2.6.21 cpu-hotplug code.

IIRC, __stop_machine_run is used by subsystems other than cpu-hotplug.
So we're not the only ones bugged.

> When process A on CPU0 try to offline the CPU1 on which the process B,
> realtime process (its task->policy == SCHED_FIFO or SCHED_RR) running
> without sleep or yield, both CPU0 and CPU1 get hang. It's because of
> the following code on __stop_machine_run().
> struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
> unsigned int cpu)
> {
> ...
> p = kthread_create(do_stop, &smdata, "kstopmachine");
> if (!IS_ERR(p)) {
> kthread_bind(p, cpu);
> wake_up_process(p);
> wait_for_completion(&smdata.done);
> }
> ...
> }
> kstopmachine is created, bound to the CPU1, and woken up here, but
> this process can't start to run because reschedule doesn't occur on
> CPU1. Hence CPU0 also be able to run because it's waiting completion
> of CPU1's offline work.

But each of these stop_machine_run threads run at MAX_RT_PRIO - 1
with SCHED_FIFO. So unless B is also running at MAX_RT_PRIO - 1,
there should not be a hang. Moreover, I doubt if we have kernel threads(B)
which runs at MAX_RT_PRIO - 1.

Nevertheless, with the freezer based approach that we're experimenting,
this problem shouldn't arise. We expect the whole system to get frozen
before we actually do a cpu_down() (which will then call
__stop_machine_run). So any such rogue RT task will have to first fail
the freezer ( which it will), but that's ok, since on a freezer-fail,
we just thaw all the processes and get the system up and running again.
Yeah, the cpu-hotplug operation will fail though.

> Thanks,
> Sat

Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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