[PATCH 5.10 62/63] sched/idle: Make the idle timer expire in hard interrupt context

From: Greg Kroah-Hartman
Date: Fri Sep 24 2021 - 09:13:18 EST

From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

[ Upstream commit 9848417926353daa59d2b05eb26e185063dbac6e ]

The intel powerclamp driver will setup a per-CPU worker with RT
priority. The worker will then invoke play_idle() in which it remains in
the idle poll loop until it is stopped by the timer it started earlier.

That timer needs to expire in hard interrupt context on PREEMPT_RT.
Otherwise the timer will expire in ksoftirqd as a SOFT timer but that task
won't be scheduled on the CPU because its priority is lower than the
priority of the worker which is in the idle loop.

Always expire the idle timer in hard interrupt context.

Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210906113034.jgfxrjdvxnjqgtmc@xxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
kernel/sched/idle.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 36b545f17206..2593a733c084 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -372,10 +372,10 @@ void play_idle_precise(u64 duration_ns, u64 latency_ns)

it.done = 0;
- hrtimer_init_on_stack(&it.timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+ hrtimer_init_on_stack(&it.timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
it.timer.function = idle_inject_timer_fn;
hrtimer_start(&it.timer, ns_to_ktime(duration_ns),

while (!READ_ONCE(it.done))