linux-kernel - [PATCH 2/2] kernel/hrtimer: Ignore slack time for RT tasks

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Date:   Mon, 23 Jan 2023 09:32:06 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     tglx@...utronix.de
Cc:     mingo@...nel.org, peterz@...radead.org, bigeasy@...utronix.de,
        dave@...olabs.net, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] kernel/hrtimer: Ignore slack time for RT tasks

While in theory the timer can be triggered before expires+delta,
for the cases of RT tasks they really have no business giving
any lenience for extra slack time, so override any passed value
by the user and always use zero for schedule_hrtimeout_range()
calls. Furthermore, this is similar to what the nanosleep(2)
family already does with current->timer_slack_ns.

Signed-off-by: Davidlohr Bueso <dave@...olabs.net>
---
 kernel/time/hrtimer.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 8336c2618ec1..78f2e07d3e7d 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -2270,7 +2270,7 @@ void __init hrtimers_init(void)
 /**
  * schedule_hrtimeout_range_clock - sleep until timeout
  * @expires:	timeout value (ktime_t)
- * @delta:	slack in expires timeout (ktime_t)
+ * @delta:	slack in expires timeout (ktime_t) for SCHED_OTHER tasks
  * @mode:	timer mode
  * @clock_id:	timer clock to be used
  */
@@ -2297,6 +2297,13 @@ schedule_hrtimeout_range_clock(ktime_t *expires, u64 delta,
 		return -EINTR;
 	}
 
+	/*
+	 * Override any slack passed by the user if under
+	 * rt contraints.
+	 */
+	if (rt_task(current))
+		delta = 0;
+
 	hrtimer_init_sleeper_on_stack(&t, clock_id, mode);
 	hrtimer_set_expires_range_ns(&t.timer, *expires, delta);
 	hrtimer_sleeper_start_expires(&t, mode);
@@ -2316,7 +2323,7 @@ EXPORT_SYMBOL_GPL(schedule_hrtimeout_range_clock);
 /**
  * schedule_hrtimeout_range - sleep until timeout
  * @expires:	timeout value (ktime_t)
- * @delta:	slack in expires timeout (ktime_t)
+ * @delta:	slack in expires timeout (ktime_t) for SCHED_OTHER tasks
  * @mode:	timer mode
  *
  * Make the current task sleep until the given expiry time has
@@ -2324,7 +2331,8 @@ EXPORT_SYMBOL_GPL(schedule_hrtimeout_range_clock);
  * the current task state has been set (see set_current_state()).
  *
  * The @delta argument gives the kernel the freedom to schedule the
- * actual wakeup to a time that is both power and performance friendly.
+ * actual wakeup to a time that is both power and performance friendly
+ * for regular (non RT/DL) tasks.
  * The kernel give the normal best effort behavior for "@expires+@...ta",
  * but may decide to fire the timer earlier, but no earlier than @expires.
  *
-- 
2.39.0