diff options
| author | Frederic Weisbecker <frederic@kernel.org> | 2023-11-14 22:38:39 +0300 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2023-11-15 11:57:51 +0300 |
| commit | dd5403869a40595eb953f12e8cd2bb57bb88bb67 (patch) | |
| tree | e33eb92c5c23d2f737b2bb1c6fa545a4a929707c /kernel | |
| parent | 63ba8422f876e32ee564ea95da9a7313b13ff0a1 (diff) | |
| download | linux-dd5403869a40595eb953f12e8cd2bb57bb88bb67.tar.xz | |
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: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Link: https://lkml.kernel.org/r/20231114193840.4041-2-frederic@kernel.org
Diffstat (limited to 'kernel')
| -rw-r--r-- | kernel/sched/idle.c | 30 |
1 files changed, 30 insertions, 0 deletions
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)) { |