[PATCH v2 4/6] sched/core: Pause KCOV in __schedule()

[Karl Mehltretter <kmehltretter@gmail.com>]

kernel/sched/ is not instrumented, but callees such as sched_clock(),
architecture CPU-capacity helpers and profile_hits() are.

During preemption and schedule() calls, instrumented callees can add
nondeterministic scheduler coverage to current.

With CONFIG_KCOV_SELFTEST added to x86_64 defconfig, the interrupt
selftest fails under QEMU, detecting spurious coverage in
arch_scale_cpu_capacity().

Annotating each callee would spread exclusions across architectures.
Pause across __schedule() instead, extending the scheduler exclusion to
its callees.

KCOV_PAUSED remains set while a task is switched out. The guard in its
resumed __schedule() frame restores the prior state.

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)()
     - clarify the test configuration and 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 3cbf817f52a4a..e49f0bfa0ca73 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7073,6 +7073,9 @@ static void __sched notrace __schedule(int sched_mode)
 	struct rq *rq;
 	int cpu;
 
+	/* Instrumented callees would leak coverage into current. */
+	guard(kcov_pause)();
+
 	/* Trace preemptions consistently with task switches */
 	trace_sched_entry_tp(sched_mode == SM_PREEMPT);
 
-- 
2.53.0