[PATCH v2 6/6] sched/core: Pause KCOV in wake_up_new_task()

[Karl Mehltretter <kmehltretter@gmail.com>]

wake_up_new_task() is uninstrumented, but CPU selection and enqueue call
instrumented helpers. During a KCOV-enabled fork, they can record
scheduler, hrtimer and clockevent coverage into the parent.

The paths depend on runqueue and CPU state, so coverage varies between
identical forks. Pause KCOV for the whole function, extending the
scheduler exclusion to new-task wakeups.

Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Notes:
    v2:
     - take guard(kcov_pause)()
     - reword the pause comment

 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e863fac02e38f..f7ddfbbb5a494 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4947,6 +4947,9 @@ void wake_up_new_task(struct task_struct *p)
 	struct rq *rq;
 	int wake_flags = WF_FORK;
 
+	/* Instrumented callees would leak coverage into current. */
+	guard(kcov_pause)();
+
 	raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
 	WRITE_ONCE(p->__state, TASK_RUNNING);
 	/*
-- 
2.53.0