[PATCH v4 5/6] sched/core: Pause KCOV in try_to_wake_up()

[Karl Mehltretter <kmehltretter@gmail.com>]

try_to_wake_up() is built without KCOV instrumentation, but it calls
helpers instrumented with KCOV, 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 the wakeup body with the kcov_pause guard. Take it before the
preemption guard so preemption is re-enabled while KCOV remains paused.
Wrapping only select_task_rq() would miss SCHED_HRTICK arming during
enqueue.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 kernel/sched/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 419dd98645bd..f19221282278 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4273,6 +4273,8 @@ bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
  */
 int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 {
+	/* Callees instrumented with KCOV would leak coverage into current. */
+	guard(kcov_pause)();
 	guard(preempt)();
 	int cpu, success = 0;
 
-- 
2.53.0