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sched/cpuidle: Comment about timers requirements VS idle handler

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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
This commit is contained in:
Frederic Weisbecker 2023-11-14 14:38:39 -05:00 committed by Peter Zijlstra
parent 63ba8422f8
commit dd5403869a
1 changed files with 30 additions and 0 deletions
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30
kernel/sched/idle.c
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@ -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)) {