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>

1 files changed, 10 insertions, 7 deletions

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();
         }