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

This reverts commit 3adfd8e344.  It causes
merge issues with 5.18-rc1 and has to be reverted for now.  If it is
still needed, it can be added back after 5.18-rc1.

Bug: 31501544
Bug: 168521633
Cc: John Dias <joaodias@google.com>
Cc: J. Avila <elavila@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I03d90db2c9169975740a1f0d3193e44f27006c31

arch/arm64/configs/gki_defconfig

@@ -30,7 +30,6 @@ CONFIG_CGROUP_CPUACCT=y
 CONFIG_CGROUP_BPF=y
 CONFIG_NAMESPACES=y
 # CONFIG_PID_NS is not set
-CONFIG_RT_SOFTINT_OPTIMIZATION=y
 # CONFIG_RD_BZIP2 is not set
 # CONFIG_RD_LZMA is not set
 # CONFIG_RD_XZ is not set

init/Kconfig

@@ -1274,17 +1274,6 @@ 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

@@ -65,29 +65,8 @@ 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 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);
-	}
-}
-#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;
@@ -124,11 +103,6 @@ static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p,
 	if (lowest_mask) {
 		cpumask_and(lowest_mask, &p->cpus_mask, 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
@@ -173,16 +147,12 @@ 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, drop_nopreempts))
+		if (!__cpupri_find(cp, p, lowest_mask, idx))
 			continue;
 
 		if (!lowest_mask || !fitness_fn)
@@ -204,17 +174,6 @@ 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

@@ -1568,22 +1568,6 @@ 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;
-}
-EXPORT_SYMBOL_GPL(task_may_not_preempt);
-#endif /* CONFIG_RT_SOFTINT_OPTIMIZATION */
-
 static int
 select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 {
@@ -1591,7 +1575,6 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 	struct rq *rq;
 	bool test;
 	int target_cpu = -1;
-	bool may_not_preempt;
 
 	trace_android_rvh_select_task_rq_rt(p, cpu, flags & 0xF,
 					flags, &target_cpu);
@@ -1608,12 +1591,7 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 	curr = READ_ONCE(rq->curr); /* unlocked access */
 
 	/*
-	 * If the current task on @p's runqueue is a softirq task,
+	 * 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.
@@ -1638,10 +1616,9 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 	 * requirement of the task - which is only important on heterogeneous
 	 * systems like big.LITTLE.
 	 */
-	may_not_preempt = task_may_not_preempt(curr, cpu);
+	test = curr &&
-	test = (curr && (may_not_preempt ||
+	       unlikely(rt_task(curr)) &&
-			 (unlikely(rt_task(curr)) &&
+	       (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio);
-			  (curr->nr_cpus_allowed < 2 || curr->prio <= p->prio))));
 
 	if (test || !rt_task_fits_capacity(p, cpu)) {
 		int target = find_lowest_rq(p);
@@ -1654,14 +1631,11 @@ select_task_rq_rt(struct task_struct *p, int cpu, int flags)
 			goto out_unlock;
 
 		/*
-		 * If cpu is non-preemptible, prefer remote cpu
+		 * Don't bother moving it if the destination CPU is
-		 * 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 &&
-		    (may_not_preempt ||
+		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
-		     p->prio < cpu_rq(target)->rt.highest_prio.curr))
 			cpu = target;
 	}
 

kernel/sched/sched.h

@@ -3161,14 +3161,3 @@ extern int sched_dynamic_mode(const char *str);
 extern void sched_dynamic_update(int mode);
 #endif
 
-/*
- * 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 */