sched: Reverse sched_class layout

Because GCC-12 is fully stupid about array bounds and it's just really
hard to get a solid array definition from a linker script, flip the
array order to avoid needing negative offsets :-/

This makes the whole relational pointer magic a little less obvious, but
alas.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/YoOLLmLG7HRTXeEm@hirez.programming.kicks-ass.net

include/asm-generic/vmlinux.lds.h

@@ -126,13 +126,13 @@
  */
 #define SCHED_DATA				\
 	STRUCT_ALIGN();				\
-	__begin_sched_classes = .;		\
-	*(__idle_sched_class)			\
-	*(__fair_sched_class)			\
-	*(__rt_sched_class)			\
-	*(__dl_sched_class)			\
+	__sched_class_highest = .;		\
 	*(__stop_sched_class)			\
-	__end_sched_classes = .;
+	*(__dl_sched_class)			\
+	*(__rt_sched_class)			\
+	*(__fair_sched_class)			\
+	*(__idle_sched_class)			\
+	__sched_class_lowest = .;
 
 /* The actual configuration determine if the init/exit sections
  * are handled as text/data or they can be discarded (which

kernel/sched/core.c

@@ -2193,7 +2193,7 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
 {
 	if (p->sched_class == rq->curr->sched_class)
 		rq->curr->sched_class->check_preempt_curr(rq, p, flags);
-	else if (p->sched_class > rq->curr->sched_class)
+	else if (sched_class_above(p->sched_class, rq->curr->sched_class))
 		resched_curr(rq);
 
 	/*
@@ -5692,7 +5692,7 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	 * higher scheduling class, because otherwise those lose the
 	 * opportunity to pull in more work from other CPUs.
 	 */
-	if (likely(prev->sched_class <= &fair_sched_class &&
+	if (likely(!sched_class_above(prev->sched_class, &fair_sched_class) &&
 		   rq->nr_running == rq->cfs.h_nr_running)) {
 
 		p = pick_next_task_fair(rq, prev, rf);
@@ -9472,11 +9472,11 @@ void __init sched_init(void)
 	int i;
 
 	/* Make sure the linker didn't screw up */
-	BUG_ON(&idle_sched_class + 1 != &fair_sched_class ||
-	       &fair_sched_class + 1 != &rt_sched_class ||
-	       &rt_sched_class + 1   != &dl_sched_class);
+	BUG_ON(&idle_sched_class != &fair_sched_class + 1 ||
+	       &fair_sched_class != &rt_sched_class + 1 ||
+	       &rt_sched_class   != &dl_sched_class + 1);
 #ifdef CONFIG_SMP
-	BUG_ON(&dl_sched_class + 1 != &stop_sched_class);
+	BUG_ON(&dl_sched_class != &stop_sched_class + 1);
 #endif
 
 	wait_bit_init();

kernel/sched/sched.h

@@ -2177,6 +2177,8 @@ static inline void set_next_task(struct rq *rq, struct task_struct *next)
  *
  *   include/asm-generic/vmlinux.lds.h
  *
+ * *CAREFUL* they are laid out in *REVERSE* order!!!
+ *
  * Also enforce alignment on the instance, not the type, to guarantee layout.
  */
 #define DEFINE_SCHED_CLASS(name) \
@@ -2185,17 +2187,16 @@ const struct sched_class name##_sched_class \
 	__section("__" #name "_sched_class")
 
 /* Defined in include/asm-generic/vmlinux.lds.h */
-extern struct sched_class __begin_sched_classes[];
-extern struct sched_class __end_sched_classes[];
-
-#define sched_class_highest (__end_sched_classes - 1)
-#define sched_class_lowest  (__begin_sched_classes - 1)
+extern struct sched_class __sched_class_highest[];
+extern struct sched_class __sched_class_lowest[];
 
 #define for_class_range(class, _from, _to) \
-	for (class = (_from); class != (_to); class--)
+	for (class = (_from); class < (_to); class++)
 
 #define for_each_class(class) \
-	for_class_range(class, sched_class_highest, sched_class_lowest)
+	for_class_range(class, __sched_class_highest, __sched_class_lowest)
+
+#define sched_class_above(_a, _b)	((_a) < (_b))
 
 extern const struct sched_class stop_sched_class;
 extern const struct sched_class dl_sched_class;