linux-kernel - [PATCH v2 1/1] hrtimer: Ignore slack time for RT tasks in hrtimer_start_range

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Message-Id: <20240805140930.29462-2-felix.moessbauer@siemens.com>
Date: Mon,  5 Aug 2024 16:09:30 +0200
From: Felix Moessbauer <felix.moessbauer@...mens.com>
To: linux-kernel@...r.kernel.org
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	jan.kiszka@...mens.com,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	qyousef@...alina.io,
	Felix Moessbauer <felix.moessbauer@...mens.com>,
	stable@...r.kernel.org
Subject: [PATCH v2 1/1] hrtimer: Ignore slack time for RT tasks in hrtimer_start_range_ns()

RT tasks do not have any timerslack, as this induces jitter. By
that, the timer slack is already ignored in the nanosleep family and
schedule_hrtimeout_range() (fixed in 0c52310f2600).

The hrtimer_start_range_ns function is indirectly used by glibc-2.33+
for timed waits on condition variables. These are sometimes used in
RT applications for realtime queue processing. At least on the
combination of kernel 5.10 and glibc-2.31, the timed wait on condition
variables in rt tasks was precise (no slack), however glibc-2.33
changed the internal wait implementation, exposing this oversight.

Make the timer slack consistent across all hrtimer programming code,
by ignoring the timerslack for tasks with rt policies also in the last
remaining location in hrtimer_start_range_ns().

Cc: stable@...r.kernel.org
Signed-off-by: Felix Moessbauer <felix.moessbauer@...mens.com>
---
 kernel/time/hrtimer.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index b8ee320208d4..e8b44e7c281f 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1274,7 +1274,7 @@ static int __hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,
  * hrtimer_start_range_ns - (re)start an hrtimer
  * @timer:	the timer to be added
  * @tim:	expiry time
- * @delta_ns:	"slack" range for the timer
+ * @delta_ns:	"slack" range for the timer for SCHED_OTHER tasks
  * @mode:	timer mode: absolute (HRTIMER_MODE_ABS) or
  *		relative (HRTIMER_MODE_REL), and pinned (HRTIMER_MODE_PINNED);
  *		softirq based mode is considered for debug purpose only!
@@ -1299,6 +1299,10 @@ void hrtimer_start_range_ns(struct hrtimer *timer, ktime_t tim,

 	base = lock_hrtimer_base(timer, &flags);

+	/* rt-tasks do not have a timer slack for obvious reasons */
+	if (task_is_realtime(current))
+		delta_ns = 0;
+
 	if (__hrtimer_start_range_ns(timer, tim, delta_ns, mode, base))
 		hrtimer_reprogram(timer, true);

-- 
2.39.2