Date: Mon, 19 Apr 2004 18:28:53 +0530
From: Srivatsa Vaddagiri <vatsa@in.ibm.com>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: rusty@au1.ibm.com, Ingo Molnar <mingo@elte.hu>, akpm@osdl.org,
       linux-kernel@vger.kernel.org, lhcs-devel@lists.sourceforge.net
Subject: Re: [lhcs-devel] Re: CPU Hotplug broken -mm5 onwards
Message-ID: <20040419125853.GB6835@in.ibm.com>
Reply-To: vatsa@in.ibm.com
References: <20040418170613.GA21769@in.ibm.com> <408348B6.7020606@yahoo.com.au>
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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

	
-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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