[PATCH v5 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.

During task-context wakeups such as a pipe write waking a reader, these
helpers record coverage that depends on the target task and runqueue
state into the waking task. Before commit feb5c9a2447a ("softirq:
Preserve interrupt context on return from interrupt"), the IRQ-exit
ktimersd wakeup under PREEMPT_RT also ran in task context and failed
CONFIG_KCOV_SELFTEST in kthread_is_per_cpu().

Pause the wakeup body with kcov_pause_scope(). 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.

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 1691eb19f31a..4bd0d5736ccf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4323,6 +4323,8 @@ 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. */
+	kcov_pause_scope();
 	guard(preempt)();
 	int cpu, success = 0;
 
-- 
2.39.5