[PATCH 4/5] sched: pause KCOV in try_to_wake_up()

[Karl Mehltretter <kmehltretter@gmail.com>]

try_to_wake_up() is uninstrumented, but it calls instrumented helpers
such as kthread_is_per_cpu(), CPU capacity helpers and SCHED_HRTICK
arming. They can record into current while in_task() is true.

CONFIG_KCOV_SELFTEST exposes this under PREEMPT_RT. The interrupt
selftest fails on x86_64 in kthread_is_per_cpu(): RT runs the timer
softirq in a thread, so the wakeup runs in task context during the
selftest's spin. The same helpers leak into non-RT syscall wakeups such
as a pipe write waking a reader.

Pause all of try_to_wake_up(). Wrapping only select_task_rq() would miss
SCHED_HRTICK arming during enqueue.

Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d5663df6c702..1e562a7ff0af 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4252,6 +4252,8 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 {
 	guard(preempt)();
 	int cpu, success = 0;
+	/* KCOV: sched/ is uninstrumented but the wakeup callees are not. */
+	unsigned int kcov_paused = kcov_pause(current);
 
 	wake_flags |= WF_TTWU;
 
@@ -4418,6 +4420,7 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	if (success)
 		ttwu_stat(p, task_cpu(p), wake_flags);
 
+	kcov_resume(current, kcov_paused);
 	return success;
 }
 
-- 
2.53.0