Re: [BUG] cpu-hotplug: Can't offline the CPU with naughty realtime processes

[Rusty Russell <rusty@rustcorp.com.au>]

On Mon, 2007-05-07 at 19:10 +0900, Satoru Takeuchi wrote:
> Hi,
> 
> I found a bug on 2.6.21 cpu-hotplug code.
> 
> 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.

Yes, we should probably move the set_scheduler call in stop_machine
(where the thread up-prioritizes itself) to before wake_up_process(p),
to avoid this happening.

Others have suggested we use the freezer; I've always distrusted that
code.  It's much trickier than stop_machine().

I look forward to your patch!
Rusty.