Re: [PATCH 2/2] selftests/sched_ext: Add lazy preemption tests

[Cheng-Yang Chou]

Hi Andrea,

On Mon, Sep 14, 2026 at 10:47:46AM +0200, Andrea Righi wrote:
> Add trace-based coverage for immediate and lazy preemption. An fexit
> probe on __resched_curr() records the requested TIF and the current task
> slice, while the stopping callback verifies that the task reaches a
> scheduling boundary.
> 
> Exercise kick requests individually, in both orders and combined in one
> call. Verify that immediate preemption and WAIT take precedence over
> lazy preemption, and that combining immediate and lazy enqueue
> preemption follows the same rule. Also test overriding the expiry
> default in both directions through scx_bpf_task_set_slice_expiry().
> 
> Extend the NO_HZ_FULL test with infinite-slice victims. Verify that lazy
> enqueue and kick requests restart a stopped tick and make forward
> progress. Keep invalid kick flag coverage and skip modes not exposed by
> the running kernel.
> 
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>

[...]

Didn't have time to go through the whole series, but I applied it on
for-next and built with PREEMPT_LAZY=y and PREEMPT_DYNAMIC=n:

	INFO: preemption mode unavailable; lazy TIF was 5, immediate TIF was 4
	ok 2 kick_lazy #
	INFO: preemption mode unavailable; lazy TIF was 5, immediate TIF was 4
	ok 4 kick_tick #
	PASSED:  6

> +static int active_lazy_mode(void)
> +{
> +	char buf[128];
> +	FILE *file;
> +
> +	file = fopen("/sys/kernel/debug/sched/preempt", "r");
> +	if (!file)
> +		return -1;

sched_init_debug() only creates that file under CONFIG_PREEMPT_DYNAMIC,
and PREEMPT_LAZY can be built without it ("select PREEMPT_BUILD if
!PREEMPT_DYNAMIC"), so the two tests pass without comparing the TIFs.

Would something like this work?

	if (!file) {
#if defined(CONFIG_PREEMPT_LAZY) && !defined(CONFIG_PREEMPT_DYNAMIC)
		return 1;
#else
		return -1;
#endif
	}

Thanks.

> +	if (!fgets(buf, sizeof(buf), file)) {
> +		fclose(file);
> +		return -1;
> +	}
> +	fclose(file);
> +
> +	if (strstr(buf, "(lazy)"))
> +		return 1;
> +	if (strstr(buf, "(full)"))
> +		return 0;
> +	return -1;
> +}
> +

-- 
Cheers,
Cheng-Yang