ANDROID: sched: avoid placing RT threads on cores handling softirqs

In certain audio use cases, scheduling RT threads on cores that are
handling softirqs can lead to glitches. Prevent this behavior.

Bug: 31501544
Bug: 168521633
Change-Id: I99dd7aaa12c11270b28dbabea484bcc8fb8ba0c1
Signed-off-by: John Dias <joaodias@google.com>
[elavila: Port to mainline, amend commit text]
Signed-off-by: J. Avila <elavila@google.com>

init/Kconfig

@@ -1211,6 +1211,17 @@ config SCHED_AUTOGROUP
 	  desktop applications.  Task group autogeneration is currently based
 	  upon task session.
 
+config RT_SOFTINT_OPTIMIZATION
+       bool "Improve RT scheduling during long softint execution"
+       depends on ARM64
+       depends on SMP
+       default n
+       help
+         Enable an optimization which tries to avoid placing RT tasks on CPUs
+	 occupied by nonpreemptible tasks, such as a long softint, or CPUs
+	 which may soon block preemptions, such as a CPU running a ksoftirq
+	 thread which handles slow softints.
+
 config SYSFS_DEPRECATED
 	bool "Enable deprecated sysfs features to support old userspace tools"
 	depends on SYSFS

kernel/sched/cpupri.c

@@ -41,8 +41,29 @@ static int convert_prio(int prio)
 	return cpupri;
 }
 
+#ifdef CONFIG_RT_SOFTINT_OPTIMIZATION
+/**
+ * drop_nopreempt_cpus - remove likely nonpreemptible cpus from the mask
+ * @lowest_mask: mask with selected CPUs (non-NULL)
+ */
+static void
+drop_nopreempt_cpus(struct cpumask *lowest_mask)
+{
+	unsigned cpu = cpumask_first(lowest_mask);
+	while (cpu < nr_cpu_ids) {
+		/* unlocked access */
+		struct task_struct *task = READ_ONCE(cpu_rq(cpu)->curr);
+		if (task_may_not_preempt(task, cpu)) {
+			cpumask_clear_cpu(cpu, lowest_mask);
+		}
+		cpu = cpumask_next(cpu, lowest_mask);
+	}
+}
+#endif
+
 static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
-				struct cpumask *lowest_mask, int idx)
+				struct cpumask *lowest_mask, int idx,
+				bool drop_nopreempts)
 {
 	struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
 	int skip = 0;
@@ -79,6 +100,11 @@ static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
 	if (lowest_mask) {
 		cpumask_and(lowest_mask, p->cpus_ptr, vec->mask);
 
+#ifdef CONFIG_RT_SOFTINT_OPTIMIZATION
+		if (drop_nopreempts)
+			drop_nopreempt_cpus(lowest_mask);
+#endif
+
 		/*
 		 * We have to ensure that we have at least one bit
 		 * still set in the array, since the map could have
@@ -123,12 +149,16 @@ int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p,
 {
 	int task_pri = convert_prio(p->prio);
 	int idx, cpu;
+	bool drop_nopreempts = task_pri <= MAX_RT_PRIO;
 
 	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 
+#ifdef CONFIG_RT_SOFTINT_OPTIMIZATION
+retry:
+#endif
 	for (idx = 0; idx < task_pri; idx++) {
 
-		if (!__cpupri_find(cp, p, lowest_mask, idx))
+		if (!__cpupri_find(cp, p, lowest_mask, idx, drop_nopreempts))
 			continue;
 
 		if (!lowest_mask || !fitness_fn)
@@ -150,6 +180,17 @@ int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p,
 		return 1;
 	}
 
+	/*
+	 * If we can't find any non-preemptible cpu's, retry so we can
+	 * find the lowest priority target and avoid priority inversion.
+	 */
+#ifdef CONFIG_RT_SOFTINT_OPTIMIZATION
+	if (drop_nopreempts) {
+		drop_nopreempts = false;
+		goto retry;
+	}
+#endif
+
 	/*
 	 * If we failed to find a fitting lowest_mask, kick off a new search
 	 * but without taking into account any fitness criteria this time.

kernel/sched/rt.c

@@ -1436,6 +1436,21 @@ static void yield_task_rt(struct rq *rq)
 #ifdef CONFIG_SMP
 static int find_lowest_rq(struct task_struct *task);
 
+#ifdef CONFIG_RT_SOFTINT_OPTIMIZATION
+/*
+ * Return whether the task on the given cpu is currently non-preemptible
+ * while handling a softirq or is likely to block preemptions soon because
+ * it is a ksoftirq thread.
+ */
+bool
+task_may_not_preempt(struct task_struct *task, int cpu)
+{
+	struct task_struct *cpu_ksoftirqd = per_cpu(ksoftirqd, cpu);
+	return (task_thread_info(task)->preempt_count & SOFTIRQ_MASK) ||
+	       task == cpu_ksoftirqd;
+}
+#endif /* CONFIG_RT_SOFTINT_OPTIMIZATION */
+
 static int
 select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 {
@@ -1443,6 +1458,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	struct rq *rq;
 	bool test;
 	int target_cpu = -1;
+	bool may_not_preempt;
 
 	trace_android_rvh_select_task_rq_rt(p, cpu, sd_flag,
 					flags, &target_cpu);
@@ -1459,7 +1475,12 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	curr = READ_ONCE(rq->curr); /* unlocked access */
 
 	/*
-	 * If the current task on @p's runqueue is an RT task, then
+	 * If the current task on @p's runqueue is a softirq task,
+	 * it may run without preemption for a time that is
+	 * ill-suited for a waiting RT task. Therefore, try to
+	 * wake this RT task on another runqueue.
+	 *
+	 * Also, if the current task on @p's runqueue is an RT task, then
 	 * try to see if we can wake this RT task up on another
 	 * runqueue. Otherwise simply start this RT task
 	 * on its current runqueue.
@@ -1484,9 +1505,10 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	 * requirement of the task - which is only important on heterogeneous
 	 * systems like big.LITTLE.
 	 */
-	test = curr &&
+	may_not_preempt = task_may_not_preempt(curr, cpu);
-	       unlikely(rt_task(curr)) &&
+	test = (curr && (may_not_preempt ||
-	       (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
+			 (unlikely(rt_task(curr)) &&
+			  (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio))));
 
 	if (test || !rt_task_fits_capacity(p, cpu)) {
 		int target = find_lowest_rq(p);
@@ -1499,11 +1521,14 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 			goto out_unlock;
 
 		/*
-		 * Don't bother moving it if the destination CPU is
+		 * If cpu is non-preemptible, prefer remote cpu
+		 * even if it's running a higher-prio task.
+		 * Otherwise: Don't bother moving it if the destination CPU is
 		 * not running a lower priority task.
 		 */
 		if (target != -1 &&
-		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
+		    (may_not_preempt ||
+		     p->prio < cpu_rq(target)->rt.highest_prio.curr))
 			cpu = target;
 	}
 

kernel/sched/sched.h

@@ -2671,3 +2671,15 @@ static inline bool is_per_cpu_kthread(struct task_struct *p)
 
 void swake_up_all_locked(struct swait_queue_head *q);
 void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);
+
+/*
+ * task_may_not_preempt - check whether a task may not be preemptible soon
+ */
+#ifdef CONFIG_RT_SOFTINT_OPTIMIZATION
+extern bool task_may_not_preempt(struct task_struct *task, int cpu);
+#else
+static inline bool task_may_not_preempt(struct task_struct *task, int cpu)
+{
+	return false;
+}
+#endif /* CONFIG_RT_SOFTINT_OPTIMIZATION */