[PATCH] cpusets+hotplug+preepmt broken

This patch removes the entwining of cpusets and hotplug code in the "No
more Mr. Nice Guy" case of sched.c move_task_off_dead_cpu().

Since the hotplug code is holding a spinlock at this point, we cannot take
the cpuset semaphore, cpuset_sem, as would seem to be required either to
update the tasks cpuset, or to scan up the nested cpuset chain, looking for
the nearest cpuset ancestor that still has some CPUs that are online. So
we just punt and blast the tasks cpus_allowed with all bits allowed.

This reverts these lines of code to what they were before the cpuset patch.
And it updates the cpuset Doc file, to match.

The one known alternative to this that seems to work came from Dinakar
Guniguntala, and required the hotplug code to take the cpuset_sem semaphore
much earlier in its processing. So far as we know, the increased locking
entanglement between cpusets and hot plug of this alternative approach is
not worth doing in this case.

Signed-off-by: Paul Jackson <[e-mail]>
Acked-by: Nathan Lynch <[e-mail]>
Acked-by: Dinakar Guniguntala <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/cpusets.txt

···
 There is an exception to the above.  If hotplug funtionality is used
 to remove all the CPUs that are currently assigned to a cpuset,
 then the kernel will automatically update the cpus_allowed of all
-tasks attached to CPUs in that cpuset with the online CPUs of the
-nearest parent cpuset that still has some CPUs online.  When memory
+tasks attached to CPUs in that cpuset to allow all CPUs.  When memory
 hotplug functionality for removing Memory Nodes is available, a
 similar exception is expected to apply there as well.  In general,
 the kernel prefers to violate cpuset placement, over starving a task

kernel/sched.c

···
 
 	/* No more Mr. Nice Guy. */
 	if (dest_cpu == NR_CPUS) {
-		tsk->cpus_allowed = cpuset_cpus_allowed(tsk);
+		cpus_setall(tsk->cpus_allowed);
 		dest_cpu = any_online_cpu(tsk->cpus_allowed);
 
 		/*