cpuset, isolcpus: document relationship between cpusets & isolcpus

Document the subtly changed relationship between cpusets and isolcpus.
Turns out the old documentation did not match the code...

Signed-off-by: Rik van Riel <[e-mail]>
Suggested-by: Peter Zijlstra <[e-mail]>
Acked-by: Zefan Li <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/cgroups/cpusets.txt

···
 than one big one, but doing so means that overloads in one of the
 two domains won't be load balanced to the other one.
 
-By default, there is one sched domain covering all CPUs, except those
-marked isolated using the kernel boot time "isolcpus=" argument.
+By default, there is one sched domain covering all CPUs, including those
+marked isolated using the kernel boot time "isolcpus=" argument. However,
+the isolated CPUs will not participate in load balancing, and will not
+have tasks running on them unless explicitly assigned.
 
 This default load balancing across all CPUs is not well suited for
 the following two situations:
···
 such partially load balanced cpusets, as they may be artificially
 constrained to some subset of the CPUs allowed to them, for lack of
 load balancing to the other CPUs.
+
+CPUs in "cpuset.isolcpus" were excluded from load balancing by the
+isolcpus= kernel boot option, and will never be load balanced regardless
+of the value of "cpuset.sched_load_balance" in any cpuset.
 
 1.7.1 sched_load_balance implementation details.
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