sched/smt: Make sched_smt_present track topology

[linux.git] / kernel / sched / core.c

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f12225f26b70a630ac185cfccba2b140f191c47e..6fedf3a98581b34b388a014d9ddb671ab8281a1f 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5738,15 +5738,10 @@ int sched_cpu_activate(unsigned int cpu)
 
 #ifdef CONFIG_SCHED_SMT
        /*
-        * The sched_smt_present static key needs to be evaluated on every
-        * hotplug event because at boot time SMT might be disabled when
-        * the number of booted CPUs is limited.
-        *
-        * If then later a sibling gets hotplugged, then the key would stay
-        * off and SMT scheduling would never be functional.
+        * When going up, increment the number of cores with SMT present.
         */
-       if (cpumask_weight(cpu_smt_mask(cpu)) > 1)
-               static_branch_enable_cpuslocked(&sched_smt_present);
+       if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
+               static_branch_inc_cpuslocked(&sched_smt_present);
 #endif
        set_cpu_active(cpu, true);
 
@@ -5790,6 +5785,14 @@ int sched_cpu_deactivate(unsigned int cpu)
         */
        synchronize_rcu_mult(call_rcu, call_rcu_sched);
 
+#ifdef CONFIG_SCHED_SMT
+       /*
+        * When going down, decrement the number of cores with SMT present.
+        */
+       if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
+               static_branch_dec_cpuslocked(&sched_smt_present);
+#endif
+
        if (!sched_smp_initialized)
                return 0;
 
@@ -5851,11 +5854,14 @@ void __init sched_init_smp(void)
        /*
         * There's no userspace yet to cause hotplug operations; hence all the
         * CPU masks are stable and all blatant races in the below code cannot
-        * happen.
+        * happen. The hotplug lock is nevertheless taken to satisfy lockdep,
+        * but there won't be any contention on it.
         */
+       cpus_read_lock();
        mutex_lock(&sched_domains_mutex);
        sched_init_domains(cpu_active_mask);
        mutex_unlock(&sched_domains_mutex);
+       cpus_read_unlock();
 
        /* Move init over to a non-isolated CPU */
        if (set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_DOMAIN)) < 0)