sched/preempt: Fix up missed PREEMPT_NEED_RESCHED folding

With various drivers wanting to inject idle time; we get people
calling idle routines outside of the idle loop proper.

Therefore we need to be extra careful about not missing
TIF_NEED_RESCHED -> PREEMPT_NEED_RESCHED propagations.

While looking at this, I also realized there's a small window in the
existing idle loop where we can miss TIF_NEED_RESCHED; when it hits
right after the tif_need_resched() test at the end of the loop but
right before the need_resched() test at the start of the loop.

So move preempt_fold_need_resched() out of the loop where we're
guaranteed to have TIF_NEED_RESCHED set.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-x9jgh45oeayzajz2mjt0y7d6@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/mwait.h

@@ -53,7 +53,7 @@ static inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 		if (!need_resched())
 			__mwait(eax, ecx);
 	}
-	__current_clr_polling();
+	current_clr_polling();
 }
 
 #endif /* _ASM_X86_MWAIT_H */

include/linux/preempt.h

@@ -134,6 +134,21 @@ do { \
 #undef preempt_check_resched
 #endif
 
+#ifdef CONFIG_PREEMPT
+#define preempt_set_need_resched() \
+do { \
+	set_preempt_need_resched(); \
+} while (0)
+#define preempt_fold_need_resched() \
+do { \
+	if (tif_need_resched()) \
+		set_preempt_need_resched(); \
+} while (0)
+#else
+#define preempt_set_need_resched() do { } while (0)
+#define preempt_fold_need_resched() do { } while (0)
+#endif
+
 #ifdef CONFIG_PREEMPT_NOTIFIERS
 
 struct preempt_notifier;

include/linux/sched.h

@@ -2745,6 +2745,21 @@ static inline bool __must_check current_clr_polling_and_test(void)
 }
 #endif
 
+static inline void current_clr_polling(void)
+{
+	__current_clr_polling();
+
+	/*
+	 * Ensure we check TIF_NEED_RESCHED after we clear the polling bit.
+	 * Once the bit is cleared, we'll get IPIs with every new
+	 * TIF_NEED_RESCHED and the IPI handler, scheduler_ipi(), will also
+	 * fold.
+	 */
+	smp_mb(); /* paired with resched_task() */
+
+	preempt_fold_need_resched();
+}
+
 static __always_inline bool need_resched(void)
 {
 	return unlikely(tif_need_resched());

kernel/cpu/idle.c

@@ -105,14 +105,17 @@ static void cpu_idle_loop(void)
 				__current_set_polling();
 			}
 			arch_cpu_idle_exit();
-			/*
-			 * We need to test and propagate the TIF_NEED_RESCHED
-			 * bit here because we might not have send the
-			 * reschedule IPI to idle tasks.
-			 */
-			if (tif_need_resched())
-				set_preempt_need_resched();
 		}
+
+		/*
+		 * Since we fell out of the loop above, we know
+		 * TIF_NEED_RESCHED must be set, propagate it into
+		 * PREEMPT_NEED_RESCHED.
+		 *
+		 * This is required because for polling idle loops we will
+		 * not have had an IPI to fold the state for us.
+		 */
+		preempt_set_need_resched();
 		tick_nohz_idle_exit();
 		schedule_preempt_disabled();
 	}

kernel/sched/core.c

@@ -1510,8 +1510,7 @@ void scheduler_ipi(void)
 	 * TIF_NEED_RESCHED remotely (for the first time) will also send
 	 * this IPI.
 	 */
-	if (tif_need_resched())
-		set_preempt_need_resched();
+	preempt_fold_need_resched();
 
 	if (llist_empty(&this_rq()->wake_list)
 			&& !tick_nohz_full_cpu(smp_processor_id())