Re: [PATCH] fork: fix default_timer_slack_ns inheritance from RT parent

[MOESSBAUER, Felix]

On Thu, 2026-08-06 at 16:20 +0800, Guanyou.Chen wrote:
> Per prctl(2), "Timer slack is not applied to threads that are scheduled
> under a real-time scheduling policy." RT tasks' timer_slack_ns is
> forcibly 0 - not a user-chosen value but a "not applicable" state.
> 
> When copy_process() sets the child's default_timer_slack_ns from the
> parent's timer_slack_ns, it inherits this forced 0 for RT parents. This
> corrupts the child's reset target, making prctl(PR_SET_TIMERSLACK, 0)
> and sched_setscheduler() back to NORMAL unable to restore a meaningful
> default.
> 
> Fix this by using default_timer_slack_ns (which preserves the pre-RT
> value) when the parent is RT/DL. For non-RT parents, timer_slack_ns is
> a meaningful user value and the existing behavior is preserved.
> 
> Fixes: ed4fb6d7ef68 ("hrtimer: Use and report correct timerslack values for realtime tasks")
> Depends-on: 63c1a12bc0e0 ("sched: restore timer_slack_ns when resetting RT policy on fork")
> Reported-by: Qiaoting.Lin <linqiaoting@xxxxxxxxxx>
> Signed-off-by: Guanyou.Chen <chenguanyou@xxxxxxxxxx>
> Signed-off-by: Chunhui.Li <chunhui.li@xxxxxxxxxxxx>
> ---
>  kernel/fork.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 65113a304518..bc4df18bfd90 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -23,6 +23,7 @@
>  #include <linux/sched/task_stack.h>
>  #include <linux/sched/cputime.h>
>  #include <linux/sched/ext.h>
> +#include <linux/sched/rt.h>
>  #include <linux/seq_file.h>
>  #include <linux/rtmutex.h>
>  #include <linux/init.h>
> @@ -2133,7 +2134,10 @@ __latent_entropy struct task_struct *copy_process(
>  	retval = -EAGAIN;
>  #endif
>  
> -	p->default_timer_slack_ns = current->timer_slack_ns;

Hi, I'm wondering why it was implemented like that before, instead of

p->default_timer_slack_ns = current->default_timer_slack_ns

along with setting the default _timer_slack_ns of the init task as
well. Currently this is needed because the init_task only sets the
timer_slack_ns, which is then copied by all forks.

That logic goes back to 6976675d94042 ("hrtimer: create a "timer_slack"
field in the task struct") from 17 years ago.

Felix

> +	if (rt_or_dl_task_policy(current))
> +		p->default_timer_slack_ns = current->default_timer_slack_ns;
> +	else
> +		p->default_timer_slack_ns = current->timer_slack_ns;
>  
>  #ifdef CONFIG_PSI
>  	p->psi_flags = 0;
> -- 
> 2.34.1