sched_ext: Make task_can_run_on_remote_rq() use common task_allowed_on_cpu()

task_can_run_on_remote_rq() is similar to is_cpu_allowed() but there are
subtle differences. It currently open codes all the tests. This is
cumbersome to understand and error-prone in case the intersecting tests need
to be updated.

Factor out the common part - testing whether the task is allowed on the CPU
at all regardless of the CPU state - into task_allowed_on_cpu() and make
both is_cpu_allowed() and SCX's task_can_run_on_remote_rq() use it. As the
code is now linked between the two and each contains only the extra tests
that differ between them, it's less error-prone when the conditions need to
be updated. Also, improve the comment to explain why they are different.

v2: Replace accidental "extern inline" with "static inline" (Peter).
Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: David Vernet <void@manifault.com>

kernel/sched/core.c

@@ -2342,7 +2342,7 @@ static inline bool rq_has_pinned_tasks(struct rq *rq)
 static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 {
 	/* When not in the task's cpumask, no point in looking further. */
-	if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+	if (!task_allowed_on_cpu(p, cpu))
 		return false;
 
 	/* migrate_disabled() must be allowed to finish. */
@@ -2351,7 +2351,7 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 
 	/* Non kernel threads are not allowed during either online or offline. */
 	if (!(p->flags & PF_KTHREAD))
-		return cpu_active(cpu) && task_cpu_possible(cpu, p);
+		return cpu_active(cpu);
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))

kernel/sched/ext.c

@@ -2224,19 +2224,30 @@ static void consume_local_task(struct rq *rq, struct scx_dispatch_q *dsq,
 
 #ifdef CONFIG_SMP
 /*
- * Similar to kernel/sched/core.c::is_cpu_allowed() but we're testing whether @p
- * can be pulled to @rq.
+ * Similar to kernel/sched/core.c::is_cpu_allowed(). However, there are two
+ * differences:
+ *
+ * - is_cpu_allowed() asks "Can this task run on this CPU?" while
+ *   task_can_run_on_remote_rq() asks "Can the BPF scheduler migrate the task to
+ *   this CPU?".
+ *
+ *   While migration is disabled, is_cpu_allowed() has to say "yes" as the task
+ *   must be allowed to finish on the CPU that it's currently on regardless of
+ *   the CPU state. However, task_can_run_on_remote_rq() must say "no" as the
+ *   BPF scheduler shouldn't attempt to migrate a task which has migration
+ *   disabled.
+ *
+ * - The BPF scheduler is bypassed while the rq is offline and we can always say
+ *   no to the BPF scheduler initiated migrations while offline.
  */
 static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq)
 {
 	int cpu = cpu_of(rq);
 
-	if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+	if (!task_allowed_on_cpu(p, cpu))
 		return false;
 	if (unlikely(is_migration_disabled(p)))
 		return false;
-	if (!(p->flags & PF_KTHREAD) && unlikely(!task_cpu_possible(cpu, p)))
-		return false;
 	if (!scx_rq_online(rq))
 		return false;
 	return true;

kernel/sched/sched.h

@@ -2534,6 +2534,19 @@ extern void sched_balance_trigger(struct rq *rq);
 extern int __set_cpus_allowed_ptr(struct task_struct *p, struct affinity_context *ctx);
 extern void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx);
 
+static inline bool task_allowed_on_cpu(struct task_struct *p, int cpu)
+{
+	/* When not in the task's cpumask, no point in looking further. */
+	if (!cpumask_test_cpu(cpu, p->cpus_ptr))
+		return false;
+
+	/* Can @cpu run a user thread? */
+	if (!(p->flags & PF_KTHREAD) && !task_cpu_possible(cpu, p))
+		return false;
+
+	return true;
+}
+
 static inline cpumask_t *alloc_user_cpus_ptr(int node)
 {
 	/*
@@ -2567,6 +2580,11 @@ extern int push_cpu_stop(void *arg);
 
 #else /* !CONFIG_SMP: */
 
+static inline bool task_allowed_on_cpu(struct task_struct *p, int cpu)
+{
+	return true;
+}
+
 static inline int __set_cpus_allowed_ptr(struct task_struct *p,
 					 struct affinity_context *ctx)
 {