Commit dd540386 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Peter Zijlstra
Browse files

sched/cpuidle: Comment about timers requirements VS idle handler 



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

Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: default avatarRafael J. Wysocki <rafael@kernel.org>
Link: https://lkml.kernel.org/r/20231114193840.4041-2-frederic@kernel.org
parent 63ba8422

kernel/sched/idle.c

+30 −0
Original line number Diff line number Diff line
@@ -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)) {