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>

5 files changed, 42 insertions, 10 deletions

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 19b71c439256..1da25a5f96f9 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/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 */
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index dd9ddf8af205..59749fc48328 100644
--- a/include/linux/preempt.h
+++ b/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;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a03875221663..ffccdad050b5 100644
--- a/include/linux/sched.h
+++ b/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());
diff --git a/kernel/cpu/idle.c b/kernel/cpu/idle.c
index 988573a9a387..277f494c2a9a 100644
--- a/kernel/cpu/idle.c
+++ b/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();
         }
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 392c6f87906e..0326c06953eb 100644
--- a/kernel/sched/core.c
+++ b/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())