sched: Fix migrate_disable() vs rt/dl balancing

In order to minimize the interference of migrate_disable() on lower
priority tasks, which can be deprived of runtime due to being stuck
below a higher priority task. Teach the RT/DL balancers to push away
these higher priority tasks when a lower priority task gets selected
to run on a freshly demoted CPU (pull).

This adds migration interference to the higher priority task, but
restores bandwidth to system that would otherwise be irrevocably lost.
Without this it would be possible to have all tasks on the system
stuck on a single CPU, each task preempted in a migrate_disable()
section with a single high priority task running.

This way we can still approximate running the M highest priority tasks
on the system.

Migrating the top task away is (ofcourse) still subject to
migrate_disable() too, which means the lower task is subject to an
interference equivalent to the worst case migrate_disable() section.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Bristot de Oliveira <bristot@redhat.com>
Link: https://lkml.kernel.org/r/20201023102347.499155098@infradead.org

4 files changed, 162 insertions, 29 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9ce2fc7d3d51..e92d7853057c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1763,11 +1763,6 @@ void migrate_enable(void)
 }
 EXPORT_SYMBOL_GPL(migrate_enable);
 
-static inline bool is_migration_disabled(struct task_struct *p)
-{
-	return p->migration_disabled;
-}
-
 static inline bool rq_has_pinned_tasks(struct rq *rq)
 {
 	return rq->nr_pinned;
@@ -1972,6 +1967,49 @@ out:
 	return 0;
 }
 
+int push_cpu_stop(void *arg)
+{
+	struct rq *lowest_rq = NULL, *rq = this_rq();
+	struct task_struct *p = arg;
+
+	raw_spin_lock_irq(&p->pi_lock);
+	raw_spin_lock(&rq->lock);
+
+	if (task_rq(p) != rq)
+		goto out_unlock;
+
+	if (is_migration_disabled(p)) {
+		p->migration_flags |= MDF_PUSH;
+		goto out_unlock;
+	}
+
+	p->migration_flags &= ~MDF_PUSH;
+
+	if (p->sched_class->find_lock_rq)
+		lowest_rq = p->sched_class->find_lock_rq(p, rq);
+
+	if (!lowest_rq)
+		goto out_unlock;
+
+	// XXX validate p is still the highest prio task
+	if (task_rq(p) == rq) {
+		deactivate_task(rq, p, 0);
+		set_task_cpu(p, lowest_rq->cpu);
+		activate_task(lowest_rq, p, 0);
+		resched_curr(lowest_rq);
+	}
+
+	double_unlock_balance(rq, lowest_rq);
+
+out_unlock:
+	rq->push_busy = false;
+	raw_spin_unlock(&rq->lock);
+	raw_spin_unlock_irq(&p->pi_lock);
+
+	put_task_struct(p);
+	return 0;
+}
+
 /*
  * sched_class::set_cpus_allowed must do the below, but is not required to
  * actually call this function.
@@ -2052,6 +2090,14 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 
 	/* Can the task run on the task's current CPU? If so, we're done */
 	if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) {
+		struct task_struct *push_task = NULL;
+
+		if ((flags & SCA_MIGRATE_ENABLE) &&
+		    (p->migration_flags & MDF_PUSH) && !rq->push_busy) {
+			rq->push_busy = true;
+			push_task = get_task_struct(p);
+		}
+
 		pending = p->migration_pending;
 		if (pending) {
 			refcount_inc(&pending->refs);
@@ -2060,6 +2106,11 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 		}
 		task_rq_unlock(rq, p, rf);
 
+		if (push_task) {
+			stop_one_cpu_nowait(rq->cpu, push_cpu_stop,
+					    p, &rq->push_work);
+		}
+
 		if (complete)
 			goto do_complete;
 
@@ -2098,6 +2149,7 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
 	if (flags & SCA_MIGRATE_ENABLE) {
 
 		refcount_inc(&pending->refs); /* pending->{arg,stop_work} */
+		p->migration_flags &= ~MDF_PUSH;
 		task_rq_unlock(rq, p, rf);
 
 		pending->arg = (struct migration_arg) {
@@ -2716,11 +2768,6 @@ static inline int __set_cpus_allowed_ptr(struct task_struct *p,
 
 static inline void migrate_disable_switch(struct rq *rq, struct task_struct *p) { }
 
-static inline bool is_migration_disabled(struct task_struct *p)
-{
-	return false;
-}
-
 static inline bool rq_has_pinned_tasks(struct rq *rq)
 {
 	return false;
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 3d3fd8370447..eed2e449b313 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2129,6 +2129,9 @@ static int push_dl_task(struct rq *rq)
 		return 0;
 
 retry:
+	if (is_migration_disabled(next_task))
+		return 0;
+
 	if (WARN_ON(next_task == rq->curr))
 		return 0;
 
@@ -2206,7 +2209,7 @@ static void push_dl_tasks(struct rq *rq)
 static void pull_dl_task(struct rq *this_rq)
 {
 	int this_cpu = this_rq->cpu, cpu;
-	struct task_struct *p;
+	struct task_struct *p, *push_task;
 	bool resched = false;
 	struct rq *src_rq;
 	u64 dmin = LONG_MAX;
@@ -2236,6 +2239,7 @@ static void pull_dl_task(struct rq *this_rq)
 			continue;
 
 		/* Might drop this_rq->lock */
+		push_task = NULL;
 		double_lock_balance(this_rq, src_rq);
 
 		/*
@@ -2267,17 +2271,27 @@ static void pull_dl_task(struct rq *this_rq)
 					   src_rq->curr->dl.deadline))
 				goto skip;
 
-			resched = true;
-
-			deactivate_task(src_rq, p, 0);
-			set_task_cpu(p, this_cpu);
-			activate_task(this_rq, p, 0);
-			dmin = p->dl.deadline;
+			if (is_migration_disabled(p)) {
+				push_task = get_push_task(src_rq);
+			} else {
+				deactivate_task(src_rq, p, 0);
+				set_task_cpu(p, this_cpu);
+				activate_task(this_rq, p, 0);
+				dmin = p->dl.deadline;
+				resched = true;
+			}
 
 			/* Is there any other task even earlier? */
 		}
 skip:
 		double_unlock_balance(this_rq, src_rq);
+
+		if (push_task) {
+			raw_spin_unlock(&this_rq->lock);
+			stop_one_cpu_nowait(src_rq->cpu, push_cpu_stop,
+					    push_task, &src_rq->push_work);
+			raw_spin_lock(&this_rq->lock);
+		}
 	}
 
 	if (resched)
@@ -2524,6 +2538,7 @@ const struct sched_class dl_sched_class
 	.rq_online              = rq_online_dl,
 	.rq_offline             = rq_offline_dl,
 	.task_woken		= task_woken_dl,
+	.find_lock_rq		= find_lock_later_rq,
 #endif
 
 	.task_tick		= task_tick_dl,
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index cf63346a07e4..c592e47cafed 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1859,7 +1859,7 @@ static struct task_struct *pick_next_pushable_task(struct rq *rq)
  * running task can migrate over to a CPU that is running a task
  * of lesser priority.
  */
-static int push_rt_task(struct rq *rq)
+static int push_rt_task(struct rq *rq, bool pull)
 {
 	struct task_struct *next_task;
 	struct rq *lowest_rq;
@@ -1873,6 +1873,34 @@ static int push_rt_task(struct rq *rq)
 		return 0;
 
 retry:
+	if (is_migration_disabled(next_task)) {
+		struct task_struct *push_task = NULL;
+		int cpu;
+
+		if (!pull || rq->push_busy)
+			return 0;
+
+		cpu = find_lowest_rq(rq->curr);
+		if (cpu == -1 || cpu == rq->cpu)
+			return 0;
+
+		/*
+		 * Given we found a CPU with lower priority than @next_task,
+		 * therefore it should be running. However we cannot migrate it
+		 * to this other CPU, instead attempt to push the current
+		 * running task on this CPU away.
+		 */
+		push_task = get_push_task(rq);
+		if (push_task) {
+			raw_spin_unlock(&rq->lock);
+			stop_one_cpu_nowait(rq->cpu, push_cpu_stop,
+					    push_task, &rq->push_work);
+			raw_spin_lock(&rq->lock);
+		}
+
+		return 0;
+	}
+
 	if (WARN_ON(next_task == rq->curr))
 		return 0;
 
@@ -1927,12 +1955,10 @@ retry:
 	deactivate_task(rq, next_task, 0);
 	set_task_cpu(next_task, lowest_rq->cpu);
 	activate_task(lowest_rq, next_task, 0);
-	ret = 1;
-
 	resched_curr(lowest_rq);
+	ret = 1;
 
 	double_unlock_balance(rq, lowest_rq);
-
 out:
 	put_task_struct(next_task);
 
@@ -1942,7 +1968,7 @@ out:
 static void push_rt_tasks(struct rq *rq)
 {
 	/* push_rt_task will return true if it moved an RT */
-	while (push_rt_task(rq))
+	while (push_rt_task(rq, false))
 		;
 }
 
@@ -2095,7 +2121,8 @@ void rto_push_irq_work_func(struct irq_work *work)
 	 */
 	if (has_pushable_tasks(rq)) {
 		raw_spin_lock(&rq->lock);
-		push_rt_tasks(rq);
+		while (push_rt_task(rq, true))
+			;
 		raw_spin_unlock(&rq->lock);
 	}
 
@@ -2120,7 +2147,7 @@ static void pull_rt_task(struct rq *this_rq)
 {
 	int this_cpu = this_rq->cpu, cpu;
 	bool resched = false;
-	struct task_struct *p;
+	struct task_struct *p, *push_task;
 	struct rq *src_rq;
 	int rt_overload_count = rt_overloaded(this_rq);
 
@@ -2167,6 +2194,7 @@ static void pull_rt_task(struct rq *this_rq)
 		 * double_lock_balance, and another CPU could
 		 * alter this_rq
 		 */
+		push_task = NULL;
 		double_lock_balance(this_rq, src_rq);
 
 		/*
@@ -2194,11 +2222,14 @@ static void pull_rt_task(struct rq *this_rq)
 			if (p->prio < src_rq->curr->prio)
 				goto skip;
 
-			resched = true;
-
-			deactivate_task(src_rq, p, 0);
-			set_task_cpu(p, this_cpu);
-			activate_task(this_rq, p, 0);
+			if (is_migration_disabled(p)) {
+				push_task = get_push_task(src_rq);
+			} else {
+				deactivate_task(src_rq, p, 0);
+				set_task_cpu(p, this_cpu);
+				activate_task(this_rq, p, 0);
+				resched = true;
+			}
 			/*
 			 * We continue with the search, just in
 			 * case there's an even higher prio task
@@ -2208,6 +2239,13 @@ static void pull_rt_task(struct rq *this_rq)
 		}
 skip:
 		double_unlock_balance(this_rq, src_rq);
+
+		if (push_task) {
+			raw_spin_unlock(&this_rq->lock);
+			stop_one_cpu_nowait(src_rq->cpu, push_cpu_stop,
+					    push_task, &src_rq->push_work);
+			raw_spin_lock(&this_rq->lock);
+		}
 	}
 
 	if (resched)
@@ -2449,6 +2487,7 @@ const struct sched_class rt_sched_class
 	.rq_offline             = rq_offline_rt,
 	.task_woken		= task_woken_rt,
 	.switched_from		= switched_from_rt,
+	.find_lock_rq		= find_lock_lowest_rq,
 #endif
 
 	.task_tick		= task_tick_rt,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 42de1406c0dc..56992aaca48e 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1057,6 +1057,8 @@ struct rq {
 #if defined(CONFIG_PREEMPT_RT) && defined(CONFIG_SMP)
 	unsigned int		nr_pinned;
 #endif
+	unsigned int		push_busy;
+	struct cpu_stop_work	push_work;
 };
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
@@ -1084,6 +1086,16 @@ static inline int cpu_of(struct rq *rq)
 #endif
 }
 
+#define MDF_PUSH	0x01
+
+static inline bool is_migration_disabled(struct task_struct *p)
+{
+#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT)
+	return p->migration_disabled;
+#else
+	return false;
+#endif
+}
 
 #ifdef CONFIG_SCHED_SMT
 extern void __update_idle_core(struct rq *rq);
@@ -1823,6 +1835,8 @@ struct sched_class {
 
 	void (*rq_online)(struct rq *rq);
 	void (*rq_offline)(struct rq *rq);
+
+	struct rq *(*find_lock_rq)(struct task_struct *p, struct rq *rq);
 #endif
 
 	void (*task_tick)(struct rq *rq, struct task_struct *p, int queued);
@@ -1918,6 +1932,24 @@ extern void trigger_load_balance(struct rq *rq);
 
 extern void set_cpus_allowed_common(struct task_struct *p, const struct cpumask *new_mask, u32 flags);
 
+static inline struct task_struct *get_push_task(struct rq *rq)
+{
+	struct task_struct *p = rq->curr;
+
+	lockdep_assert_held(&rq->lock);
+
+	if (rq->push_busy)
+		return NULL;
+
+	if (p->nr_cpus_allowed == 1)
+		return NULL;
+
+	rq->push_busy = true;
+	return get_task_struct(p);
+}
+
+extern int push_cpu_stop(void *arg);
+
 #endif
 
 #ifdef CONFIG_CPU_IDLE