[PATCH 1/2] sched/cpuidle: Comment about timers requirements VS idle handler

[Frederic Weisbecker <frederic@kernel.org>]

Add missing explanation concerning IRQs re-enablement constraints in
the cpuidle path against timers.

Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/sched/idle.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 565f8374ddbb..31231925f1ec 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -258,6 +258,36 @@ static void do_idle(void)
 	while (!need_resched()) {
 		rmb();
 
+		/*
+		 * Interrupts shouldn't be re-enabled from that point on until
+		 * the CPU sleeping instruction is reached. Otherwise an interrupt
+		 * may fire and queue a timer that would be ignored until the CPU
+		 * wakes from the sleeping instruction. And testing need_resched()
+		 * doesn't tell about pending needed timer reprogram.
+		 *
+		 * Several cases to consider:
+		 *
+		 * - SLEEP-UNTIL-PENDING-INTERRUPT based instructions such as
+		 *   "wfi" or "mwait" are fine because they can be entered with
+		 *   interrupt disabled.
+		 *
+		 * - sti;mwait() couple is fine because the interrupts are
+		 *   re-enabled only upon the execution of mwait, leaving no gap
+		 *   in-between.
+		 *
+		 * - ROLLBACK based idle handlers with the sleeping instruction
+		 *   called with interrupts enabled are NOT fine. In this scheme
+		 *   when the interrupt detects it has interrupted an idle handler,
+		 *   it rolls back to its beginning which performs the
+		 *   need_resched() check before re-executing the sleeping
+		 *   instruction. This can leak a pending needed timer reprogram.
+		 *   If such a scheme is really mandatory due to the lack of an
+		 *   appropriate CPU sleeping instruction, then a FAST-FORWARD
+		 *   must instead be applied: when the interrupt detects it has
+		 *   interrupted an idle handler, it must resume to the end of
+		 *   this idle handler so that the generic idle loop is iterated
+		 *   again to reprogram the tick.
+		 */
 		local_irq_disable();
 
 		if (cpu_is_offline(cpu)) {
-- 
2.42.1