Commit b39c4fab authored May 20, 2005 by Paul Jackson Committed by Linus Torvalds May 20, 2005

[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 <pj@sgi.com>
Acked-by: Nathan Lynch <ntl@pobox.com>
Acked-by: Dinakar Guniguntala <dino@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent b2665f92

Documentation/cpusets.txt

+1 −2

Original line number	Diff line number	Diff line
		@@ -252,8 +252,7 @@ in a tasks processor placement.
		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

+1 −1

Original line number	Diff line number	Diff line
		@@ -4243,7 +4243,7 @@ static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *tsk)

		/* 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);

		/*

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