Re: [RFC][PATCH] avoid cpu hot remove of cpus which have special RT tasks.

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andi Kleen wrote:
> On Friday 16 June 2006 12:26, KAMEZAWA Hiroyuki wrote:
> 
>>On 16 Jun 2006 11:14:57 +0200
>>Andi Kleen <ak@suse.de> wrote:
>>
>>
>>>KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> writes:
>>>
>>>>This is a bit pessimistic. But forecd migration of RT task which is bounded
>>>>to the special cpu will cause unpredictable trouble, I think.
>>>
>>>More trouble than running it on a CPU that is about to fail?
>>>Doubtful.
>>>
>>
>>With my patch, RT tasks will continute to run.
> 
> 
> That's the problem - if the CPU is failing and you have to remove
> it the task will likely corrupt its data or fail in other ways
> if it doesn't allow it.
> 
> Better to let RT tasks run a little slower on another CPU.
> 
>  
> 
>>Assume there are some multi-threaded tasks with SCHED_FIFO.
>>If they uses some kind of synchronization in user land and task is migrated to
>>other cpus, it will cause dead-lock.
> 
> 
> If its CPU fails much worse things than that will happen.
> 
> One way might be to break affinity of all processes in the system on hot unplug
> - then your deadlock would be avoided - but it might be a bit radical.

Agreed. The kernel is just doing some basic fallback behaviour. If you
actually have a critical RT system, you probably need to have much more
sophisticated handling of CPU unplug anyway. So it doesn't make much
sense to complicate the kernel for this.

-- 
SUSE Labs, Novell Inc.
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