Re: [lhcs-devel] Re: CPU Hotplug broken -mm5 onwards

[Nick Piggin]

Srivatsa Vaddagiri wrote:
> On Mon, Apr 19, 2004 at 01:34:14PM +1000, Nick Piggin wrote:
>
> > I think a rwsem might be a good idea anyway, because
> > sched_migrate_task can end up being called pretty often with
> > balance on exec and balance on clone. The semaphore could easily
> > place undue serialisation on that path.
>
>
> I found that r/w sem does not help here ..It can still lead to deadlocks.
> One example I hit is :
>
> cpu_up takes write lock, sends out CPU_UP_PREPARE notification. As part
> of it, many do kthread_create, which uses workqueue. The work function
> is never processed because keventd would be blocked on a previous 
> work function, waiting for hotplug sem in exec path.
>
> So, as Rusty said, I think we really need to consider removing
> lock_cpu_hotplug from sched_migrate_task. AFAICS that lock
> was needed to prevent adding tasks to dead cpus. The same 
> can be accomplished by removing lock_cpu_hotplug from sched_migrate_task
> and adding a cpu_is_offline check in __migrate_task.
> This will eliminate all the deadlocks I have been hitting.

Yes this would be a better idea. Care to send Andrew a patch
against -mm?

> > Can we arrange some of these checks to disappear when HOTPLUG_CPU
> > is not set? For example, make cpu_is_offline only valid to call for
> > CPUs that have been online sometime, and can evaluate to 0 if
> > HOTPLUG_CPU is not set?
>
>
> I think this is already being done in include/linux/cpu.h

Yes I see. I didn't realise the first one was under an ifdef :P
Sorry.
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