topology.c::get_group() relies on the assumption that non-NUMA domains do
not partially overlap. Zeng Tao pointed out in [1] that such topology
descriptions, while completely bogus, can end up being exposed to the
scheduler.

In his example (8 CPUs, 2-node system), we end up with:
  MC span for CPU3 == 3-7
  MC span for CPU4 == 4-7

The first pass through get_group(3, sdd@MC) will result in the following
sched_group list:

  3 -> 4 -> 5 -> 6 -> 7
  ^                  /
   `----------------'

And a later pass through get_group(4, sdd@MC) will "corrupt" that to:

  3 -> 4 -> 5 -> 6 -> 7
       ^             /
	`-----------'

which will completely break things like 'while (sg != sd->groups)' when
using CPU3's base sched_domain.

There already are some architecture-specific checks in place such as
x86/kernel/smpboot.c::topology.sane(), but this is something we can detect
in the core scheduler, so it seems worthwhile to do so.

Warn and abort the construction of the sched domains if such a broken
topology description is detected. Note that this is somewhat
expensive (O(t.c²), 't' non-NUMA topology levels and 'c' CPUs) and could be
gated under SCHED_DEBUG if deemed necessary.

Testing
=======

Dietmar managed to reproduce this using the following qemu incantation:

  $ qemu-system-aarch64 -kernel ./Image -hda ./qemu-image-aarch64.img \
  -append 'root=/dev/vda console=ttyAMA0 loglevel=8 sched_debug' -smp \
  cores=8 --nographic -m 512 -cpu cortex-a53 -machine virt -numa \
  node,cpus=0-2,nodeid=0 -numa node,cpus=3-7,nodeid=1

alongside the following drivers/base/arch_topology.c hack (AIUI wouldn't be
needed if '-smp cores=X, sockets=Y' would work with qemu):

8<---
@@ -465,6 +465,9 @@ void update_siblings_masks(unsigned int cpuid)
 		if (cpuid_topo->package_id != cpu_topo->package_id)
 			continue;

+		if ((cpu < 4 && cpuid > 3) || (cpu > 3 && cpuid < 4))
+			continue;
+
 		cpumask_set_cpu(cpuid, &cpu_topo->core_sibling);
 		cpumask_set_cpu(cpu, &cpuid_topo->core_sibling);

8<---

[1]: https://lkml.kernel.org/r/1577088979-8545-1-git-send-email-prime.zeng@hisilicon.comReported-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200115160915.22575-1-valentin.schneider@arm.com