[PATCH] sched/topology: Disable sched_asym_cpucapacity on domain destruction

[Valentin Schneider <valentin.schneider@arm.com>]

While the static key is correctly initialized as being disabled, it will
remain forever enabled once turned on. This means that if we start with an
asymmetric system and hotplug out enough CPUs to end up with an SMP system,
the static key will remain set - which is obviously wrong. We should detect
this and turn off things like misfit migration and EAS wakeups.

Having that key enabled should also mandate

  per_cpu(sd_asym_cpucapacity, cpu) != NULL

for all CPUs, but this is obviously not true with the above.

On top of that, sched domain rebuilds first lead to attaching the NULL
domain to the affected CPUs, which means there will be a window where the
static key is set but the sd_asym_cpucapacity shortcut points to NULL even
if asymmetry hasn't been hotplugged out.

Disable the static key when destroying domains, and let
build_sched_domains() (re) enable it as needed.

Cc: <stable@vger.kernel.org>
Fixes: df054e8445a4 ("sched/topology: Add static_key for asymmetric CPU capacity optimizations")
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/sched/topology.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index b5667a273bf6..c49ae57a0611 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2123,7 +2123,8 @@ static void detach_destroy_domains(const struct cpumask *cpu_map)
 {
 	int i;
 
+	static_branch_disable_cpuslocked(&sched_asym_cpucapacity);
+
 	rcu_read_lock();
 	for_each_cpu(i, cpu_map)
 		cpu_attach_domain(NULL, &def_root_domain, i);
--
2.22.0