On Mon, 14 Oct 2019 at 13:47, Valentin Schneider valentin.schneider@arm.com wrote:
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
Acked-by: Vincent Guittot <vincent .guittot@linaro.org>
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