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 can't just disable the key on domain destruction, we need to count how many asymmetric root domains we have.

Consider the following example using Juno r0 which is 2+4 big.LITTLE, where two identical cpusets are created: they both span both big and LITTLE CPUs:

asym0 asym1 [ ][ ] L L B L L B

cgcreate -g cpuset:asym0 cgset -r cpuset.cpus=0,1,3 asym0 cgset -r cpuset.mems=0 asym0 cgset -r cpuset.cpu_exclusive=1 asym0

cgcreate -g cpuset:asym1 cgset -r cpuset.cpus=2,4,5 asym1 cgset -r cpuset.mems=0 asym1 cgset -r cpuset.cpu_exclusive=1 asym1

cgset -r cpuset.sched_load_balance=0 .

(the CPU numbering may look odd because on the Juno LITTLEs are CPUs 0,3-5 and bigs are CPUs 1-2)

If we make one of those SMP (IOW remove asymmetry) by e.g. hotplugging its big core, we would end up with an SMP cpuset and an asymmetric cpuset - the static key must remain set, because we still have one asymmetric root domain.

With the above example, this could be done with:

echo 0 > /sys/devices/system/cpu/cpu2/online

Which would result in:

asym0 asym1 [ ][ ] L L B L L

When both SMP and asymmetric cpusets are present, all CPUs will observe sched_asym_cpucapacity being set (it is system-wide), but not all CPUs observe asymmetry in their sched domain hierarchy:

per_cpu(sd_asym_cpucapacity, <any CPU in asym0>) == <some SD at DIE level> per_cpu(sd_asym_cpucapacity, <any CPU in asym1>) == NULL

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") Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com 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 2e7af755e17a..6ec1e595b1d4 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", @@ -2121,9 +2121,12 @@ int sched_init_domains(const struct cpumask *cpu_map) */ static void detach_destroy_domains(const struct cpumask *cpu_map) { + unsigned int cpu = cpumask_any(cpu_map); int i;

+ 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) cpu_attach_domain(NULL, &def_root_domain, i); -- 2.22.0