[PATCH] sched/fork: Fix timer_slack_ns inheritance for RT tasks

[Xiao Feng via B4 Relay]

From: Xiao Feng <xiaofeng5@xxxxxxxxxx>

Since commit ed4fb6d7ef68 ("hrtimer: Use and report correct timerslack
values for realtime tasks"), RT tasks have their timer_slack_ns set to 0
in __setscheduler_params(). This introduces two related problems when RT
tasks fork children that end up running as CFS:

Problem 1: sched_reset_on_fork

When a RT task with sched_reset_on_fork set forks a child:

1. dup_task_struct() copies timer_slack_ns (0) from the RT parent.
2. copy_process() sets:
     p->default_timer_slack_ns = current->timer_slack_ns (= 0)
3. sched_fork() demotes the child to SCHED_NORMAL but does not
   restore timer_slack_ns.

Result: CFS child has timer_slack_ns = 0 and default_timer_slack_ns = 0
permanently.

Problem 2: RT fork followed by later policy change

When a RT task forks a child (without reset_on_fork), the child inherits
RT policy with timer_slack_ns = 0. copy_process() sets
default_timer_slack_ns = current->timer_slack_ns (= 0). If the child is
later demoted to CFS via sched_setscheduler(), __setscheduler_params()
tries to restore timer_slack_ns from default_timer_slack_ns, but it is 0.

Result: same as above.

Both problems prevent timer coalescing for these CFS tasks, causing
unnecessary wakeups and increased power consumption. Writing 0 to
/proc/pid/timerslack_ns also cannot restore a proper default.

Fix both issues:

1. In copy_process(), inherit default_timer_slack_ns from the parent's
   default_timer_slack_ns (which is preserved across RT transitions)
   instead of timer_slack_ns (which is 0 for RT tasks).

2. In sched_fork(), when sched_reset_on_fork demotes RT/DL to CFS,
   explicitly restore timer_slack_ns from the parent's
   default_timer_slack_ns, falling back to 50us if it is also 0.

Fixes: ed4fb6d7ef68 ("hrtimer: Use and report correct timerslack values for realtime tasks")
Signed-off-by: Xiao Feng <xiaofeng5@xxxxxxxxxx>
---
 kernel/fork.c       | 2 +-
 kernel/sched/core.c | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 5f3fdfdb14c7..c3ef5ebb3037 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2174,7 +2174,7 @@ __latent_entropy struct task_struct *copy_process(
 	retval = -EAGAIN;
 #endif
 
-	p->default_timer_slack_ns = current->timer_slack_ns;
+	p->default_timer_slack_ns = current->default_timer_slack_ns;
 
 #ifdef CONFIG_PSI
 	p->psi_flags = 0;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b8871449d3c6..9b63560a45de 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4710,6 +4710,8 @@ 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 = current->default_timer_slack_ns ?: 50000;
+			p->default_timer_slack_ns = p->timer_slack_ns;
 		} else if (PRIO_TO_NICE(p->static_prio) < 0)
 			p->static_prio = NICE_TO_PRIO(0);
 

---
base-commit: 27fa82620cbaa89a7fc11ac3057701d598813e87
change-id: 20260520-sched-fork-fix-timer-slack-6dfacb033d43

Best regards,
--  
Xiao Feng <xiaofeng5@xxxxxxxxxx>