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@vger.kernel.org Signed-off-by: Felix Moessbauer felix.moessbauer@siemens.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);