[tip: sched/core] sched: Switch wait_task_inactive to HRTIMER_MODE_REL_HARD

[tip-bot2 for Sebastian Andrzej Siewior]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     e9e5ba93a24d946d6e70d5c85f74254335a6555b
Gitweb:        https://git.kernel.org/tip/e9e5ba93a24d946d6e70d5c85f74254335a6555b
Author:        Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate:    Thu, 26 Aug 2021 19:04:08 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 09 Sep 2021 11:27:30 +02:00

sched: Switch wait_task_inactive to HRTIMER_MODE_REL_HARD

With PREEMPT_RT enabled all hrtimers callbacks will be invoked in
softirq mode unless they are explicitly marked as HRTIMER_MODE_HARD.
During boot kthread_bind() is used for the creation of per-CPU threads
and then hangs in wait_task_inactive() if the ksoftirqd is not
yet up and running.
The hang disappeared since commit
   26c7295be0c5e ("kthread: Do not preempt current task if it is going to call schedule()")

but enabling function trace on boot reliably leads to the freeze on boot
behaviour again.
The timer in wait_task_inactive() can not be directly used by a user
interface to abuse it and create a mass wake up of several tasks at the
same time leading to long sections with disabled interrupts.
Therefore it is safe to make the timer HRTIMER_MODE_REL_HARD.

Switch the timer to HRTIMER_MODE_REL_HARD.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210826170408.vm7rlj7odslshwch@xxxxxxxxxxxxx
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a22cc3c..d19d1ba 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3234,7 +3234,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
 			ktime_t to = NSEC_PER_SEC / HZ;
 
 			set_current_state(TASK_UNINTERRUPTIBLE);
-			schedule_hrtimeout(&to, HRTIMER_MODE_REL);
+			schedule_hrtimeout(&to, HRTIMER_MODE_REL_HARD);
 			continue;
 		}