sched: Fix balance vs hotplug race

Since (e761b77: cpu hotplug, sched: Introduce cpu_active_map and redo
sched domain managment) we have cpu_active_mask which is suppose to rule
scheduler migration and load-balancing, except it never (fully) did.

The particular problem being solved here is a crash in try_to_wake_up()
where select_task_rq() ends up selecting an offline cpu because
select_task_rq_fair() trusts the sched_domain tree to reflect the
current state of affairs, similarly select_task_rq_rt() trusts the
root_domain.

However, the sched_domains are updated from CPU_DEAD, which is after the
cpu is taken offline and after stop_machine is done. Therefore it can
race perfectly well with code assuming the domains are right.

Cure this by building the domains from cpu_active_mask on
CPU_DOWN_PREPARE.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

4 files changed, 41 insertions, 27 deletions

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 789cf5f920ce..d77b54733c5b 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -84,6 +84,7 @@ extern const struct cpumask *const cpu_active_mask;
 #define num_online_cpus()       cpumask_weight(cpu_online_mask)
 #define num_possible_cpus()     cpumask_weight(cpu_possible_mask)
 #define num_present_cpus()      cpumask_weight(cpu_present_mask)
+#define num_active_cpus()       cpumask_weight(cpu_active_mask)
 #define cpu_online(cpu)         cpumask_test_cpu((cpu), cpu_online_mask)
 #define cpu_possible(cpu)       cpumask_test_cpu((cpu), cpu_possible_mask)
 #define cpu_present(cpu)        cpumask_test_cpu((cpu), cpu_present_mask)
@@ -92,6 +93,7 @@ extern const struct cpumask *const cpu_active_mask;
 #define num_online_cpus()       1
 #define num_possible_cpus()     1
 #define num_present_cpus()      1
+#define num_active_cpus()       1
 #define cpu_online(cpu)         ((cpu) == 0)
 #define cpu_possible(cpu)       ((cpu) == 0)
 #define cpu_present(cpu)        ((cpu) == 0)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6ba0f1ecb212..b21688640377 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -212,6 +212,8 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
         err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
                                         hcpu, -1, &nr_calls);
         if (err == NOTIFY_BAD) {
+                set_cpu_active(cpu, true);
+
                 nr_calls--;
                 __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED | mod,
                                           hcpu, nr_calls, NULL);
@@ -223,11 +225,11 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 
         /* Ensure that we are not runnable on dying cpu */
         cpumask_copy(old_allowed, &current->cpus_allowed);
-        set_cpus_allowed_ptr(current,
-                             cpumask_of(cpumask_any_but(cpu_online_mask, cpu)));
+        set_cpus_allowed_ptr(current, cpu_active_mask);
 
         err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
         if (err) {
+                set_cpu_active(cpu, true);
                 /* CPU didn't die: tell everyone.  Can't complain. */
                 if (raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED | mod,
                                             hcpu) == NOTIFY_BAD)
@@ -292,9 +294,6 @@ int __ref cpu_down(unsigned int cpu)
 
         err = _cpu_down(cpu, 0);
 
-        if (cpu_online(cpu))
-                set_cpu_active(cpu, true);
-
 out:
         cpu_maps_update_done();
         stop_machine_destroy();
@@ -387,6 +386,15 @@ int disable_nonboot_cpus(void)
          * with the userspace trying to use the CPU hotplug at the same time
          */
         cpumask_clear(frozen_cpus);
+
+        for_each_online_cpu(cpu) {
+                if (cpu == first_cpu)
+                        continue;
+                set_cpu_active(cpu, false);
+        }
+
+        synchronize_sched();
+
         printk("Disabling non-boot CPUs ...\n");
         for_each_online_cpu(cpu) {
                 if (cpu == first_cpu)
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 43fb7e800028..ba401fab459f 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -872,7 +872,7 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
                 if (retval < 0)
                         return retval;
 
-                if (!cpumask_subset(trialcs->cpus_allowed, cpu_online_mask))
+                if (!cpumask_subset(trialcs->cpus_allowed, cpu_active_mask))
                         return -EINVAL;
         }
         retval = validate_change(cs, trialcs);
@@ -2010,7 +2010,7 @@ static void scan_for_empty_cpusets(struct cpuset *root)
                 }
 
                 /* Continue past cpusets with all cpus, mems online */
-                if (cpumask_subset(cp->cpus_allowed, cpu_online_mask) &&
+                if (cpumask_subset(cp->cpus_allowed, cpu_active_mask) &&
                     nodes_subset(cp->mems_allowed, node_states[N_HIGH_MEMORY]))
                         continue;
 
@@ -2019,7 +2019,7 @@ static void scan_for_empty_cpusets(struct cpuset *root)
                 /* Remove offline cpus and mems from this cpuset. */
                 mutex_lock(&callback_mutex);
                 cpumask_and(cp->cpus_allowed, cp->cpus_allowed,
-                            cpu_online_mask);
+                            cpu_active_mask);
                 nodes_and(cp->mems_allowed, cp->mems_allowed,
                                                 node_states[N_HIGH_MEMORY]);
                 mutex_unlock(&callback_mutex);
@@ -2057,8 +2057,10 @@ static int cpuset_track_online_cpus(struct notifier_block *unused_nb,
         switch (phase) {
         case CPU_ONLINE:
         case CPU_ONLINE_FROZEN:
-        case CPU_DEAD:
-        case CPU_DEAD_FROZEN:
+        case CPU_DOWN_PREPARE:
+        case CPU_DOWN_PREPARE_FROZEN:
+        case CPU_DOWN_FAILED:
+        case CPU_DOWN_FAILED_FROZEN:
                 break;
 
         default:
@@ -2067,7 +2069,7 @@ static int cpuset_track_online_cpus(struct notifier_block *unused_nb,
 
         cgroup_lock();
         mutex_lock(&callback_mutex);
-        cpumask_copy(top_cpuset.cpus_allowed, cpu_online_mask);
+        cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
         mutex_unlock(&callback_mutex);
         scan_for_empty_cpusets(&top_cpuset);
         ndoms = generate_sched_domains(&doms, &attr);
@@ -2114,7 +2116,7 @@ static int cpuset_track_online_nodes(struct notifier_block *self,
 
 void __init cpuset_init_smp(void)
 {
-        cpumask_copy(top_cpuset.cpus_allowed, cpu_online_mask);
+        cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
         top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];
 
         hotcpu_notifier(cpuset_track_online_cpus, 0);
diff --git a/kernel/sched.c b/kernel/sched.c
index aa31244caa9f..281da29d0801 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4134,7 +4134,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
         unsigned long flags;
         struct cpumask *cpus = __get_cpu_var(load_balance_tmpmask);
 
-        cpumask_copy(cpus, cpu_online_mask);
+        cpumask_copy(cpus, cpu_active_mask);
 
         /*
          * When power savings policy is enabled for the parent domain, idle
@@ -4297,7 +4297,7 @@ load_balance_newidle(int this_cpu, struct rq *this_rq, struct sched_domain *sd)
         int all_pinned = 0;
         struct cpumask *cpus = __get_cpu_var(load_balance_tmpmask);
 
-        cpumask_copy(cpus, cpu_online_mask);
+        cpumask_copy(cpus, cpu_active_mask);
 
         /*
          * When power savings policy is enabled for the parent domain, idle
@@ -4694,7 +4694,7 @@ int select_nohz_load_balancer(int stop_tick)
                 cpumask_set_cpu(cpu, nohz.cpu_mask);
 
                 /* time for ilb owner also to sleep */
-                if (cpumask_weight(nohz.cpu_mask) == num_online_cpus()) {
+                if (cpumask_weight(nohz.cpu_mask) == num_active_cpus()) {
                         if (atomic_read(&nohz.load_balancer) == cpu)
                                 atomic_set(&nohz.load_balancer, -1);
                         return 0;
@@ -7093,7 +7093,7 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
         int ret = 0;
 
         rq = task_rq_lock(p, &flags);
-        if (!cpumask_intersects(new_mask, cpu_online_mask)) {
+        if (!cpumask_intersects(new_mask, cpu_active_mask)) {
                 ret = -EINVAL;
                 goto out;
         }
@@ -7115,7 +7115,7 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
         if (cpumask_test_cpu(task_cpu(p), new_mask))
                 goto out;
 
-        if (migrate_task(p, cpumask_any_and(cpu_online_mask, new_mask), &req)) {
+        if (migrate_task(p, cpumask_any_and(cpu_active_mask, new_mask), &req)) {
                 /* Need help from migration thread: drop lock and wait. */
                 struct task_struct *mt = rq->migration_thread;
 
@@ -7269,19 +7269,19 @@ static void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 
 again:
         /* Look for allowed, online CPU in same node. */
-        for_each_cpu_and(dest_cpu, nodemask, cpu_online_mask)
+        for_each_cpu_and(dest_cpu, nodemask, cpu_active_mask)
                 if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
                         goto move;
 
         /* Any allowed, online CPU? */
-        dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_online_mask);
+        dest_cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
         if (dest_cpu < nr_cpu_ids)
                 goto move;
 
         /* No more Mr. Nice Guy. */
         if (dest_cpu >= nr_cpu_ids) {
                 cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
-                dest_cpu = cpumask_any_and(cpu_online_mask, &p->cpus_allowed);
+                dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
 
                 /*
                  * Don't tell them about moving exiting tasks or
@@ -7310,7 +7310,7 @@ move:
  */
 static void migrate_nr_uninterruptible(struct rq *rq_src)
 {
-        struct rq *rq_dest = cpu_rq(cpumask_any(cpu_online_mask));
+        struct rq *rq_dest = cpu_rq(cpumask_any(cpu_active_mask));
         unsigned long flags;
 
         local_irq_save(flags);
@@ -7564,7 +7564,7 @@ static ctl_table *sd_alloc_ctl_cpu_table(int cpu)
 static struct ctl_table_header *sd_sysctl_header;
 static void register_sched_domain_sysctl(void)
 {
-        int i, cpu_num = num_online_cpus();
+        int i, cpu_num = num_possible_cpus();
         struct ctl_table *entry = sd_alloc_ctl_entry(cpu_num + 1);
         char buf[32];
 
@@ -7574,7 +7574,7 @@ static void register_sched_domain_sysctl(void)
         if (entry == NULL)
                 return;
 
-        for_each_online_cpu(i) {
+        for_each_possible_cpu(i) {
                 snprintf(buf, 32, "cpu%d", i);
                 entry->procname = kstrdup(buf, GFP_KERNEL);
                 entry->mode = 0555;
@@ -9100,7 +9100,7 @@ match1:
         if (doms_new == NULL) {
                 ndoms_cur = 0;
                 doms_new = &fallback_doms;
-                cpumask_andnot(doms_new[0], cpu_online_mask, cpu_isolated_map);
+                cpumask_andnot(doms_new[0], cpu_active_mask, cpu_isolated_map);
                 WARN_ON_ONCE(dattr_new);
         }
 
@@ -9231,8 +9231,10 @@ static int update_sched_domains(struct notifier_block *nfb,
         switch (action) {
         case CPU_ONLINE:
         case CPU_ONLINE_FROZEN:
-        case CPU_DEAD:
-        case CPU_DEAD_FROZEN:
+        case CPU_DOWN_PREPARE:
+        case CPU_DOWN_PREPARE_FROZEN:
+        case CPU_DOWN_FAILED:
+        case CPU_DOWN_FAILED_FROZEN:
                 partition_sched_domains(1, NULL, NULL);
                 return NOTIFY_OK;
 
@@ -9279,7 +9281,7 @@ void __init sched_init_smp(void)
 #endif
         get_online_cpus();
         mutex_lock(&sched_domains_mutex);
-        arch_init_sched_domains(cpu_online_mask);
+        arch_init_sched_domains(cpu_active_mask);
         cpumask_andnot(non_isolated_cpus, cpu_possible_mask, cpu_isolated_map);
         if (cpumask_empty(non_isolated_cpus))
                 cpumask_set_cpu(smp_processor_id(), non_isolated_cpus);