On Mon, Aug 05 2024 at 14:41, Felix Moessbauer wrote:
> 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 the kernel bug.
That's hardly a bug. It's an oversight.
> This patch makes the timer slack consistent across all hrtimer
"This patch" ....
> programming code, by ignoring the timerslack for rt tasks also in the
> last remaining location in hrtimer_start_range_ns().
>
> Similar to 0c52310f2600, this fix should be backported as well.
This is not part of the change log.
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Felix Moessbauer <felix.moessbauer@xxxxxxxxxxx>
> ---
> 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 2b1469f61d9c..1b26e095114d 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 (rt_task(current))
> + delta_ns = 0;
task_is_realtime()
please
Thanks,
tglx
