Messages in this thread | | | Date | Mon, 19 Apr 2004 18:28:53 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: [lhcs-devel] Re: CPU Hotplug broken -mm5 onwards |
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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.
> 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
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Thanks and Regards, Srivatsa Vaddagiri, Linux Technology Center, IBM Software Labs, Bangalore, INDIA - 560017 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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