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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 11, 2026 at 05:41:09PM +0200, Karl Mehltretter wrote:
> kernel/sched/ is not instrumented, 

It bloody well is; there is no noinstr here, just a notrace.

> 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. */

This comment makes about as much sense as your Changelog.

> +	guard(kcov_pause)();
> +
>  	/* Trace preemptions consistently with task switches */
>  	trace_sched_entry_tp(sched_mode == SM_PREEMPT);
>  
> -- 
> 2.53.0