On Tuesday 15 Oct 2019 at 11:29:56 (+0100), Valentin Schneider 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 capacity aware wakeups.
As Quentin pointed out, having separate root domains makes this slightly trickier. We could have exclusive cpusets that create an SMP island - IOW, the domains within this root domain will not see any asymmetry. This means we need to count how many asymmetric root domains we have.
Change the simple key enablement to an increment, and decrement the key counter when destroying domains that cover asymmetric CPUs.
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
Changes since v1:
Use static_branch_{inc,dec} rather than enable/disable
kernel/sched/topology.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index b5667a273bf6..79944e969bcf 100644 --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -2026,7 +2026,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att rcu_read_unlock(); if (has_asym)
static_branch_enable_cpuslocked(&sched_asym_cpucapacity);
static_branch_inc_cpuslocked(&sched_asym_cpucapacity);if (rq && sched_debug_enabled) { pr_info("root domain span: %*pbl (max cpu_capacity = %lu)\n", @@ -2124,8 +2124,17 @@ static void detach_destroy_domains(const struct cpumask *cpu_map) int i; rcu_read_lock();
- if (static_key_enabled(&sched_asym_cpucapacity)) {
unsigned int cpu = cpumask_any(cpu_map);if (rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu)))static_branch_dec_cpuslocked(&sched_asym_cpucapacity);
Lockdep should scream for this :)
- }
- for_each_cpu(i, cpu_map) cpu_attach_domain(NULL, &def_root_domain, i);
- rcu_read_unlock();
} -- 2.22.0