sched: avoid scheduling RT threads on cores currently handling softirqs

Bug: 31501544
Change-Id: I99dd7aaa12c11270b28dbabea484bcc8fb8ba0c1
Git-commit: 080ea011fd9f47315e1fc53185872ef813b59d00
Git-repo: https://android.googlesource.com/kernel/msm
[pkondeti@codeaurora.org: resolved minor merge conflicts and fixed
checkpatch warnings]
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>

3 files changed, 77 insertions, 8 deletions

diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
index 981fcd7dc394..1d00cf8c00fa 100644
--- a/kernel/sched/cpupri.c
+++ b/kernel/sched/cpupri.c
@@ -27,6 +27,8 @@
  *  of the License.
  */
 
+#include "sched.h"
+
 #include <linux/gfp.h>
 #include <linux/sched.h>
 #include <linux/sched/rt.h>
@@ -51,6 +53,27 @@ static int convert_prio(int prio)
 }
 
 /**
+ * drop_nopreempt_cpus - remove a cpu from the mask if it is likely
+ *			 non-preemptible
+ * @lowest_mask: mask with selected CPUs (non-NULL)
+ */
+static void
+drop_nopreempt_cpus(struct cpumask *lowest_mask)
+{
+	unsigned int 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);
+	}
+}
+
+/**
  * cpupri_find - find the best (lowest-pri) CPU in the system
  * @cp: The cpupri context
  * @p: The task
@@ -70,9 +93,11 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 {
 	int idx = 0;
 	int task_pri = convert_prio(p->prio);
+	bool drop_nopreempts = task_pri <= MAX_RT_PRIO;
 
 	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 
+retry:
 	for (idx = 0; idx < task_pri; idx++) {
 		struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
 		int skip = 0;
@@ -108,7 +133,8 @@ int cpupri_find(struct cpupri *cp, struct task_struct *p,
 
 		if (lowest_mask) {
 			cpumask_and(lowest_mask, &p->cpus_allowed, vec->mask);
-
+			if (drop_nopreempts)
+				drop_nopreempt_cpus(lowest_mask);
 			/*
 			 * We have to ensure that we have at least one bit
 			 * still set in the array, since the map could have
@@ -123,7 +149,14 @@ int cpupri_find(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.
+	 */
+	if (drop_nopreempts) {
+		drop_nopreempts = false;
+		goto retry;
+	}
 	return 0;
 }
 
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 29345ed74069..338d019d0f25 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -5,6 +5,7 @@
 
 #include "sched.h"
 
+#include <linux/interrupt.h>
 #include <linux/slab.h>
 #include <linux/irq_work.h>
 #include <trace/events/sched.h>
@@ -1456,11 +1457,26 @@ select_task_rq_rt_hmp(struct task_struct *p, int cpu, int sd_flag, int flags)
 }
 #endif
 
+/*
+ * 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;
+}
+
 static int
 select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 {
 	struct task_struct *curr;
 	struct rq *rq;
+	bool may_not_preempt;
 
 #ifdef CONFIG_SCHED_HMP
 	return select_task_rq_rt_hmp(p, cpu, sd_flag, flags);
@@ -1476,7 +1492,17 @@ 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
+	 * 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.
@@ -1497,17 +1523,22 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	 * This test is optimistic, if we get it wrong the load-balancer
 	 * will have to sort it out.
 	 */
-	if (curr && unlikely(rt_task(curr)) &&
+	may_not_preempt = task_may_not_preempt(curr, cpu);
+	if (may_not_preempt ||
+	    (unlikely(rt_task(curr)) &&
 	    (curr->nr_cpus_allowed < 2 ||
-	     curr->prio <= p->prio)) {
+	     curr->prio <= p->prio))) {
 		int target = find_lowest_rq(p);
 
 		/*
-		 * Don't bother moving it if the destination CPU is
-		 * not running a lower priority task.
+		 * 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;
 	}
 	rcu_read_unlock();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a6733b57bcbc..1580af0f0a35 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2671,6 +2671,11 @@ static inline void double_rq_unlock(struct rq *rq1, struct rq *rq2)
 		__release(rq2->lock);
 }
 
+/*
+ * task_may_not_preempt - check whether a task may not be preemptible soon
+ */
+extern bool task_may_not_preempt(struct task_struct *task, int cpu);
+
 #else /* CONFIG_SMP */
 
 /*