On 15/10/2019 17:42, 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.

Could you make the example a little bit more obvious so we'll remember later the exact testcase?

We start with 2 asym (hmp) (CPU capacity) exclusive cpusets and we turn one into a sym (smp) exclusive cpuset via CPU hp.

This patch w/ extra debug on Juno [CPU0 - CPU5] = [L b b L L L]:

root@juno:~# ./scripts/create_excl_cpusets3 [ 32.898483] detach_destroy_domains(): sched_asym_cpucapacity is disabled cpu_map=0-5 [ 32.906255] build_sched_domains(): sched_asym_cpucapacity is enabled cpu_map=0-1,3 [ 32.913813] build_sched_domains(): sched_asym_cpucapacity is enabled cpu_map=2,4-5 root@juno:~# echo 0 > /sys/devices/system/cpu/cpu1/online [ 62.709591] IRQ 2: no longer affine to CPU1 [ 62.713840] CPU1: shutdown [ 62.716525] psci: CPU1 killed. root@juno:~# [ 62.728779] detach_destroy_domains(): sched_asym_cpucapacity still enabled cpu_map=0-1,3 <-- Your v1 would just have disabled sched_asym_cpucapacity here.

A hint that systems with a mix of asym and sym CPU capacity rd's have to live with the fact that specific asym code is also enabled for the CPUs of the smp rd's wouldn't harm here.

Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com

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

---

kernel/sched/topology.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c index 9318acf1d1fe..f0e730143380 100644 --- a/kernel/sched/topology.c +++ b/kernel/sched/topology.c @@ -2029,7 +2029,7 @@ 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", @@ -2125,7 +2125,10 @@ int sched_init_domains(const struct cpumask *cpu_map) static void detach_destroy_domains(const struct cpumask *cpu_map) { int i;

• unsigned int cpu = cpumask_any(cpu_map);
• if (rcu_access_pointer(per_cpu(sd_asym_cpucapacity, cpu)))

• static_branch_dec_cpuslocked(&sched_asym_cpucapacity);
  

rcu_read_lock(); for_each_cpu(i, cpu_map) -- 2.22.0