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

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Message-ID: <87a5hr5dcp.ffs@tglx>
Date: Mon, 05 Aug 2024 15:05:42 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Felix Moessbauer <felix.moessbauer@...mens.com>,
 linux-kernel@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>, Anna-Maria Behnsen
 <anna-maria@...utronix.de>, jan.kiszka@...mens.com, Felix Moessbauer
 <felix.moessbauer@...mens.com>, stable@...r.kernel.org, Sebastian Andrzej
 Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH 2/2] hrtimer: Ignore slack time for RT tasks in
 hrtimer_start_range_ns()

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@...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 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