Re: [Experimental CPU Hotplug PATCH] - Move migrate_all_tasks to CPU_DEAD handling

From: Nick Piggin <nickpiggin@yahoo.com.au>
To: vatsa@in.ibm.com
Cc: rusty@au1.ibm.com, mingo@elte.hu, akpm@osdl.org,
	linux-kernel@vger.kernel.org, lhcs-devel@lists.sourceforge.net
Subject: Re: [Experimental CPU Hotplug PATCH] - Move migrate_all_tasks to CPU_DEAD handling
Date: Wed, 07 Apr 2004 01:04:10 +1000	[thread overview]
Message-ID: <4072C6EA.1070803@yahoo.com.au> (raw)
In-Reply-To: <20040406145616.GB8516@in.ibm.com>

Srivatsa Vaddagiri wrote:
> On Tue, Apr 06, 2004 at 07:26:06PM +1000, Nick Piggin wrote:
> 
>>Also in my patch, the offline check should probably be done below
>>the check for if (cpu == this_cpu... because that should be a common
>>route.
> 
> 
> 	Will this be true for wakeups which are triggered from 
> expiring timers also? The timers on the dead CPU are migrated to other CPUs. 
> When they fire, the timer fn runs on a different CPU and can try to wake up a
> task 'n add it to dead cpu! So we probably need a unconditional cpu_is_offline
> check in try_to_wake_up?
> 

AFAIKS, no.

If this happens before migrate_all_tasks, there shouldn't be a
problem because migrate_all_tasks will move the woken task anyway.

It can't happen after migrate_all_tasks, because there is nothing
on the offline CPU to be woken up.

If you do need the check there, then my lazy migrate method is
unquestionably better, because this is the only thing it would
otherwise have to add to a fastpath. Right?