Re: [PATCH] sched: restore timer_slack_ns when resetting RT policy on fork

[Cunlong Li]

On Thu, May 21, 2026 at 10:52:50AM +0800, Guanyou.Chen wrote:
> Commit ed4fb6d7ef68 ("hrtimer: Use and report correct timerslack values
> for realtime tasks") sets timer_slack_ns to 0 for RT tasks in
> __setscheduler_params(). However, when an RT task with SCHED_RESET_ON_FORK
> creates child threads, the children inherit timer_slack_ns=0 from the
> parent. sched_fork() resets the child's policy to SCHED_NORMAL but does
> not restore timer_slack_ns, leaving the child permanently running with
> zero slack.
> 
> Additionally, init_task never initialized default_timer_slack_ns, so all
> processes in the system have default_timer_slack_ns=0 inherited from init.
> The original fork code masked this by using timer_slack_ns (50000) as the
> source for default_timer_slack_ns. After ed4fb6d7ef68, RT tasks have
> timer_slack_ns=0, exposing this latent bug.
> 
> This causes unnecessary timer interrupts and increased power consumption,
> as NORMAL threads with slack=0 prevent timer coalescing.
> 
> Fix this by:
> 1. Initializing default_timer_slack_ns=50000 in init_task.
> 2. In copy_process(), removing the incorrect default_timer_slack_ns
>    override (dup_task_struct already copies both timer_slack_ns and
>    default_timer_slack_ns correctly from the parent).
> 3. In sched_fork(), restoring timer_slack_ns from default_timer_slack_ns
>    when resetting from RT/DL to NORMAL policy.
> 
> Before this fix (RT parent, RESET_ON_FORK, 32 child threads usleep(1)):
>   child slack=0, avg_sleep=38us, ~832K interrupts/s
> 
> After this fix:
>   child slack=50000, avg_sleep=88us, ~363K interrupts/s
> 
> Fixes: 6976675d9404 ("hrtimer: create a "timer_slack" field in the task struct")
> Fixes: ed4fb6d7ef68 ("hrtimer: Use and report correct timerslack values for realtime tasks")
> Reported-by: Qiaoting.Lin <linqiaoting@xxxxxxxxxx>
> Signed-off-by: Guanyou.Chen <chenguanyou@xxxxxxxxxx>
> Signed-off-by: Chunhui.Li <chunhui.li@xxxxxxxxxxxx>
> ---
>  init/init_task.c    | 1 +
>  kernel/fork.c       | 2 --
>  kernel/sched/core.c | 1 +
>  3 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/init/init_task.c b/init/init_task.c
> index 5c838757fc10..57ff8dae9bfb 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -170,6 +170,7 @@ struct task_struct init_task __aligned(L1_CACHE_BYTES) = {
>  	INIT_CPU_TIMERS(init_task)
>  	.pi_lock	= __RAW_SPIN_LOCK_UNLOCKED(init_task.pi_lock),
>  	.timer_slack_ns = 50000, /* 50 usec default slack */
> +	.default_timer_slack_ns = 50000, /* 50 usec default slack */
>  	.thread_pid	= &init_struct_pid,
>  	.thread_node	= LIST_HEAD_INIT(init_signals.thread_head),
>  #ifdef CONFIG_AUDIT
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 65113a304518..8358df80e11d 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2133,8 +2133,6 @@ __latent_entropy struct task_struct *copy_process(
>  	retval = -EAGAIN;
>  #endif
>  
> -	p->default_timer_slack_ns = current->timer_slack_ns;
Hi Guanyou,

This changes behavior for normal (non-RT) tasks. If a process calls
prctl(PR_SET_TIMERSLACK, 200000) and then forks, the child currently
gets default_timer_slack_ns=200000 (the parent's effective slack).
With this removal, the child would get default_timer_slack_ns=50000
(the parent's original default), so a subsequent PR_SET_TIMERSLACK(0)
in the child would reset to a different value than before.
I think the fix should be narrowed to only handle the RT parent case:
	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;
Thanks
> -
>  #ifdef CONFIG_PSI
>  	p->psi_flags = 0;
>  #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b7f77c165a6e..b1a241810ce0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4649,6 +4649,7 @@ int sched_fork(u64 clone_flags, struct task_struct *p)
>  			p->policy = SCHED_NORMAL;
>  			p->static_prio = NICE_TO_PRIO(0);
>  			p->rt_priority = 0;
> +			p->timer_slack_ns = p->default_timer_slack_ns;
>  		} else if (PRIO_TO_NICE(p->static_prio) < 0)
>  			p->static_prio = NICE_TO_PRIO(0);
>  
> -- 
> 2.34.1
>