sched/topology, cpuset: Avoid spurious/wrong domain rebuilds

When disabling cpuset.sched_load_balance we expect to be able to online
CPUs without generating sched_domains. However this is currently
completely broken.

What happens is that we generate the sched_domains and then destroy
them. This is because of the spurious 'default' domain build in
cpuset_update_active_cpus(). That builds a single machine wide domain
and then schedules a work to build the 'real' domains. The work then
finds there are _no_ domains and destroys the lot again.

Furthermore, if there actually were cpusets, building the machine wide
domain is actively wrong, because it would allow tasks to 'escape' their
cpuset. Also I don't think its needed, the scheduler really should
respect the active mask.

Reported-by: Ofer Levi(SW) <oferle@mellanox.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vineet.Gupta1@synopsys.com <Vineet.Gupta1@synopsys.com>
Cc: rusty@rustcorp.com.au <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/cgroup/cpuset.c

@@ -2343,13 +2343,7 @@ void cpuset_update_active_cpus(void)
 	 * We're inside cpu hotplug critical region which usually nests
 	 * inside cgroup synchronization.  Bounce actual hotplug processing
 	 * to a work item to avoid reverse locking order.
-	 *
-	 * We still need to do partition_sched_domains() synchronously;
-	 * otherwise, the scheduler will get confused and put tasks to the
-	 * dead CPU.  Fall back to the default single domain.
-	 * cpuset_hotplug_workfn() will rebuild it as necessary.
 	 */
-	partition_sched_domains(1, NULL, NULL);
 	schedule_work(&cpuset_hotplug_work);
 }