sched: Simplify cpu-hot-unplug task migration

While discussing the need for sched_idle_next(), Oleg remarked that
since try_to_wake_up() ensures sleeping tasks will end up running on a
sane cpu, we can do away with migrate_live_tasks().

If we then extend the existing hack of migrating current from
CPU_DYING to migrating the full rq worth of tasks from CPU_DYING, the
need for the sched_idle_next() abomination disappears as well, since
idle will be the only possible thread left after the migration thread
stops.

This greatly simplifies the hot-unplug task migration path, as can be
seen from the resulting code reduction (and about half the new lines
are comments).

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1289851597.2109.547.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 67 insertions, 158 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3cd70cf91fd..29d953abb5a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1871,14 +1871,11 @@ extern void sched_clock_idle_sleep_event(void);
 extern void sched_clock_idle_wakeup_event(u64 delta_ns);
 
 #ifdef CONFIG_HOTPLUG_CPU
-extern void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p);
 extern void idle_task_exit(void);
 #else
 static inline void idle_task_exit(void) {}
 #endif
 
-extern void sched_idle_next(void);
-
 #if defined(CONFIG_NO_HZ) && defined(CONFIG_SMP)
 extern void wake_up_idle_cpu(int cpu);
 #else
diff --git a/kernel/cpu.c b/kernel/cpu.c
index f6e726f1849..8615aa65d92 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -189,7 +189,6 @@ static inline void check_for_tasks(int cpu)
 }
 
 struct take_cpu_down_param {
-        struct task_struct *caller;
         unsigned long mod;
         void *hcpu;
 };
@@ -208,11 +207,6 @@ static int __ref take_cpu_down(void *_param)
 
         cpu_notify(CPU_DYING | param->mod, param->hcpu);
 
-        if (task_cpu(param->caller) == cpu)
-                move_task_off_dead_cpu(cpu, param->caller);
-        /* Force idle task to run as soon as we yield: it should
-           immediately notice cpu is offline and die quickly. */
-        sched_idle_next();
         return 0;
 }
 
@@ -223,7 +217,6 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
         void *hcpu = (void *)(long)cpu;
         unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
         struct take_cpu_down_param tcd_param = {
-                .caller = current,
                 .mod = mod,
                 .hcpu = hcpu,
         };
@@ -253,9 +246,12 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
         }
         BUG_ON(cpu_online(cpu));
 
-        /* Wait for it to sleep (leaving idle task). */
-        while (!idle_cpu(cpu))
-                yield();
+        /*
+         * The migration_call() CPU_DYING callback will have removed all
+         * runnable tasks from the cpu, there's only the idle task left now
+         * that the migration thread is done doing the stop_machine thing.
+         */
+        BUG_ON(!idle_cpu(cpu));
 
         /* This actually kills the CPU. */
         __cpu_die(cpu);
diff --git a/kernel/sched.c b/kernel/sched.c
index 41f18695b73..b0d5f1b24a3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2366,18 +2366,15 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
                 return dest_cpu;
 
         /* No more Mr. Nice Guy. */
-        if (unlikely(dest_cpu >= nr_cpu_ids)) {
-                dest_cpu = cpuset_cpus_allowed_fallback(p);
-                /*
-                 * Don't tell them about moving exiting tasks or
-                 * kernel threads (both mm NULL), since they never
-                 * leave kernel.
-                 */
-                if (p->mm && printk_ratelimit()) {
-                        printk(KERN_INFO "process %d (%s) no "
-                               "longer affine to cpu%d\n",
-                               task_pid_nr(p), p->comm, cpu);
-                }
+        dest_cpu = cpuset_cpus_allowed_fallback(p);
+        /*
+         * Don't tell them about moving exiting tasks or
+         * kernel threads (both mm NULL), since they never
+         * leave kernel.
+         */
+        if (p->mm && printk_ratelimit()) {
+                printk(KERN_INFO "process %d (%s) no longer affine to cpu%d\n",
+                                task_pid_nr(p), p->comm, cpu);
         }
 
         return dest_cpu;
@@ -5712,29 +5709,20 @@ static int migration_cpu_stop(void *data)
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
+
 /*
- * Figure out where task on dead CPU should go, use force if necessary.
+ * Ensures that the idle task is using init_mm right before its cpu goes
+ * offline.
  */
-void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
+void idle_task_exit(void)
 {
-        struct rq *rq = cpu_rq(dead_cpu);
-        int needs_cpu, uninitialized_var(dest_cpu);
-        unsigned long flags;
+        struct mm_struct *mm = current->active_mm;
 
-        local_irq_save(flags);
+        BUG_ON(cpu_online(smp_processor_id()));
 
-        raw_spin_lock(&rq->lock);
-        needs_cpu = (task_cpu(p) == dead_cpu) && (p->state != TASK_WAKING);
-        if (needs_cpu)
-                dest_cpu = select_fallback_rq(dead_cpu, p);
-        raw_spin_unlock(&rq->lock);
-        /*
-         * It can only fail if we race with set_cpus_allowed(),
-         * in the racer should migrate the task anyway.
-         */
-        if (needs_cpu)
-                __migrate_task(p, dead_cpu, dest_cpu);
-        local_irq_restore(flags);
+        if (mm != &init_mm)
+                switch_mm(mm, &init_mm, current);
+        mmdrop(mm);
 }
 
 /*
@@ -5747,128 +5735,69 @@ void move_task_off_dead_cpu(int dead_cpu, struct task_struct *p)
 static void migrate_nr_uninterruptible(struct rq *rq_src)
 {
         struct rq *rq_dest = cpu_rq(cpumask_any(cpu_active_mask));
-        unsigned long flags;
 
-        local_irq_save(flags);
-        double_rq_lock(rq_src, rq_dest);
         rq_dest->nr_uninterruptible += rq_src->nr_uninterruptible;
         rq_src->nr_uninterruptible = 0;
-        double_rq_unlock(rq_src, rq_dest);
-        local_irq_restore(flags);
-}
-
-/* Run through task list and migrate tasks from the dead cpu. */
-static void migrate_live_tasks(int src_cpu)
-{
-        struct task_struct *p, *t;
-
-        read_lock(&tasklist_lock);
-
-        do_each_thread(t, p) {
-                if (p == current)
-                        continue;
-
-                if (task_cpu(p) == src_cpu)
-                        move_task_off_dead_cpu(src_cpu, p);
-        } while_each_thread(t, p);
-
-        read_unlock(&tasklist_lock);
 }
 
 /*
- * Schedules idle task to be the next runnable task on current CPU.
- * It does so by boosting its priority to highest possible.
- * Used by CPU offline code.
+ * remove the tasks which were accounted by rq from calc_load_tasks.
  */
-void sched_idle_next(void)
+static void calc_global_load_remove(struct rq *rq)
 {
-        int this_cpu = smp_processor_id();
-        struct rq *rq = cpu_rq(this_cpu);
-        struct task_struct *p = rq->idle;
-        unsigned long flags;
-
-        /* cpu has to be offline */
-        BUG_ON(cpu_online(this_cpu));
-
-        /*
-         * Strictly not necessary since rest of the CPUs are stopped by now
-         * and interrupts disabled on the current cpu.
-         */
-        raw_spin_lock_irqsave(&rq->lock, flags);
-
-        __setscheduler(rq, p, SCHED_FIFO, MAX_RT_PRIO-1);
-
-        activate_task(rq, p, 0);
-
-        raw_spin_unlock_irqrestore(&rq->lock, flags);
+        atomic_long_sub(rq->calc_load_active, &calc_load_tasks);
+        rq->calc_load_active = 0;
 }
 
 /*
- * Ensures that the idle task is using init_mm right before its cpu goes
- * offline.
+ * Migrate all tasks from the rq, sleeping tasks will be migrated by
+ * try_to_wake_up()->select_task_rq().
+ *
+ * Called with rq->lock held even though we'er in stop_machine() and
+ * there's no concurrency possible, we hold the required locks anyway
+ * because of lock validation efforts.
  */
-void idle_task_exit(void)
-{
-        struct mm_struct *mm = current->active_mm;
-
-        BUG_ON(cpu_online(smp_processor_id()));
-
-        if (mm != &init_mm)
-                switch_mm(mm, &init_mm, current);
-        mmdrop(mm);
-}
-
-/* called under rq->lock with disabled interrupts */
-static void migrate_dead(unsigned int dead_cpu, struct task_struct *p)
+static void migrate_tasks(unsigned int dead_cpu)
 {
         struct rq *rq = cpu_rq(dead_cpu);
-
-        /* Must be exiting, otherwise would be on tasklist. */
-        BUG_ON(!p->exit_state);
-
-        /* Cannot have done final schedule yet: would have vanished. */
-        BUG_ON(p->state == TASK_DEAD);
-
-        get_task_struct(p);
+        struct task_struct *next, *stop = rq->stop;
+        int dest_cpu;
 
         /*
-         * Drop lock around migration; if someone else moves it,
-         * that's OK. No task can be added to this CPU, so iteration is
-         * fine.
+         * Fudge the rq selection such that the below task selection loop
+         * doesn't get stuck on the currently eligible stop task.
+         *
+         * We're currently inside stop_machine() and the rq is either stuck
+         * in the stop_machine_cpu_stop() loop, or we're executing this code,
+         * either way we should never end up calling schedule() until we're
+         * done here.
          */
-        raw_spin_unlock_irq(&rq->lock);
-        move_task_off_dead_cpu(dead_cpu, p);
-        raw_spin_lock_irq(&rq->lock);
-
-        put_task_struct(p);
-}
-
-/* release_task() removes task from tasklist, so we won't find dead tasks. */
-static void migrate_dead_tasks(unsigned int dead_cpu)
-{
-        struct rq *rq = cpu_rq(dead_cpu);
-        struct task_struct *next;
+        rq->stop = NULL;
 
         for ( ; ; ) {
-                if (!rq->nr_running)
+                /*
+                 * There's this thread running, bail when that's the only
+                 * remaining thread.
+                 */
+                if (rq->nr_running == 1)
                         break;
+
                 next = pick_next_task(rq);
-                if (!next)
-                        break;
+                BUG_ON(!next);
                 next->sched_class->put_prev_task(rq, next);
-                migrate_dead(dead_cpu, next);
 
+                /* Find suitable destination for @next, with force if needed. */
+                dest_cpu = select_fallback_rq(dead_cpu, next);
+                raw_spin_unlock(&rq->lock);
+
+                __migrate_task(next, dead_cpu, dest_cpu);
+
+                raw_spin_lock(&rq->lock);
         }
-}
 
-/*
- * remove the tasks which were accounted by rq from calc_load_tasks.
- */
-static void calc_global_load_remove(struct rq *rq)
-{
-        atomic_long_sub(rq->calc_load_active, &calc_load_tasks);
-        rq->calc_load_active = 0;
+        rq->stop = stop;
 }
+
 #endif /* CONFIG_HOTPLUG_CPU */
 
 #if defined(CONFIG_SCHED_DEBUG) && defined(CONFIG_SYSCTL)
@@ -6078,15 +6007,13 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
         unsigned long flags;
         struct rq *rq = cpu_rq(cpu);
 
-        switch (action) {
+        switch (action & ~CPU_TASKS_FROZEN) {
 
         case CPU_UP_PREPARE:
-        case CPU_UP_PREPARE_FROZEN:
                 rq->calc_load_update = calc_load_update;
                 break;
 
         case CPU_ONLINE:
-        case CPU_ONLINE_FROZEN:
                 /* Update our root-domain */
                 raw_spin_lock_irqsave(&rq->lock, flags);
                 if (rq->rd) {
@@ -6098,30 +6025,19 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
                 break;
 
 #ifdef CONFIG_HOTPLUG_CPU
-        case CPU_DEAD:
-        case CPU_DEAD_FROZEN:
-                migrate_live_tasks(cpu);
-                /* Idle task back to normal (off runqueue, low prio) */
-                raw_spin_lock_irq(&rq->lock);
-                deactivate_task(rq, rq->idle, 0);
-                __setscheduler(rq, rq->idle, SCHED_NORMAL, 0);
-                rq->idle->sched_class = &idle_sched_class;
-                migrate_dead_tasks(cpu);
-                raw_spin_unlock_irq(&rq->lock);
-                migrate_nr_uninterruptible(rq);
-                BUG_ON(rq->nr_running != 0);
-                calc_global_load_remove(rq);
-                break;
-
         case CPU_DYING:
-        case CPU_DYING_FROZEN:
                 /* Update our root-domain */
                 raw_spin_lock_irqsave(&rq->lock, flags);
                 if (rq->rd) {
                         BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
                         set_rq_offline(rq);
                 }
+                migrate_tasks(cpu);
+                BUG_ON(rq->nr_running != 1); /* the migration thread */
                 raw_spin_unlock_irqrestore(&rq->lock, flags);
+
+                migrate_nr_uninterruptible(rq);
+                calc_global_load_remove(rq);
                 break;
 #endif
         }