Re: [PATCH 3/5] sched: pause KCOV in __schedule()

[Bradley Morgan]

On 7 August 2026 21:50:25 BST, Karl Mehltretter <kmehltretter@xxxxxxxxx>
wrote:
>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, the interrupt selftest fails on x86_64
>defconfig under QEMU, detecting spurious coverage in
>arch_scale_cpu_capacity(). On arm64 the same class of leak appears in
>sched_clock(), once the separate arm64 interrupt-accounting leak is
>suppressed.
>
>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. Its resumed
>__schedule() frame restores the prior state.
>
>Fixes: 5c9a8750a640 ("kernel: add kcov code coverage")
>Assisted-by: Claude:claude-opus-4-8
>Assisted-by: Claude:claude-fable-5

Awesomesauce! 

Reviewed-by: Bradley Morgan <include@xxxxxxxxx>

well, no idea how one patch could be assisted by two AI models, but I
could see some sort of Frankenstein scenario, maybe fable planning opus
implementation?, tbh idc because this patch is good anyway, thanks Karl.

>Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
>---
> kernel/sched/core.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
>diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>index b6a8fbbdd538..d5663df6c702 100644
>--- a/kernel/sched/core.c
>+++ b/kernel/sched/core.c
>@@ -7069,10 +7069,14 @@ static void __sched notrace __schedule(int sched_mode)
> 	bool is_switch = false;
> 	unsigned long *switch_count;
> 	unsigned long prev_state;
>+	unsigned int kcov_paused;
> 	struct rq_flags rf;
> 	struct rq *rq;
> 	int cpu;
> 
>+	/* KCOV: sched/ is uninstrumented but the __schedule() callees are not. */
>+	kcov_paused = kcov_pause(current);
>+
> 	/* Trace preemptions consistently with task switches */
> 	trace_sched_entry_tp(sched_mode == SM_PREEMPT);
> 
>@@ -7239,6 +7243,7 @@ static void __sched notrace __schedule(int sched_mode)
> 		raw_spin_rq_unlock_irq(rq);
> 	}
> 	trace_sched_exit_tp(is_switch);
>+	kcov_resume(current, kcov_paused);
> }
> 
> void __noreturn do_task_dead(void)
>

Thanks!