Re: [PATCH sched_ext/for-6.20] selftests/sched_ext: Fix init_enable_count flakiness

[Andrea Righi <arighi@nvidia.com>]

On Mon, Feb 02, 2026 at 09:40:22AM -1000, Tejun Heo wrote:
> The init_enable_count test is flaky. The test forks 1024 children before
> attaching the scheduler to verify that existing tasks get ops.init_task()
> called. The children were using sleep(1) before exiting.
> 
> 7900aa699c34 ("sched_ext: Fix cgroup exit ordering by moving sched_ext_free()
> to finish_task_switch()") changed when tasks are removed from scx_tasks -
> previously when the task_struct was freed, now immediately in
> finish_task_switch() when the task dies.
> 
> Before the commit, pre-forked children would linger on scx_tasks until freed
> regardless of when they exited, so the scheduler would always see them during
> iteration. The sleep(1) was unnecessary. After the commit, children are
> removed as soon as they die. The sleep(1) masks the problem in most cases but
> the test becomes flaky depending on timing.
> 
> Fix by synchronizing properly using a pipe. All children block on read() and
> the parent signals them to exit by closing the write end after attaching the
> scheduler. The children are auto-reaped so there's no need to wait on them.
> 
> Reported-by: Ihor Solodrai <ihor.solodrai@linux.dev>
> Cc: David Vernet <void@manifault.com>
> Cc: Andrea Righi <arighi@nvidia.com>
> Cc: Changwoo Min <changwoo@igalia.com>
> Cc: Emil Tsalapatis <emil@etsalapatis.com>
> Signed-off-by: Tejun Heo <tj@kernel.org>

LGTM.

Acked-by: Andrea Righi <arighi@nvidia.com>

-Andrea

> ---
>  tools/testing/selftests/sched_ext/init_enable_count.c |   34 ++++++++++++------
>  1 file changed, 23 insertions(+), 11 deletions(-)
> 
> --- a/tools/testing/selftests/sched_ext/init_enable_count.c
> +++ b/tools/testing/selftests/sched_ext/init_enable_count.c
> @@ -4,6 +4,7 @@
>   * Copyright (c) 2023 David Vernet <dvernet@meta.com>
>   * Copyright (c) 2023 Tejun Heo <tj@kernel.org>
>   */
> +#include <signal.h>
>  #include <stdio.h>
>  #include <unistd.h>
>  #include <sched.h>
> @@ -23,6 +24,9 @@ static enum scx_test_status run_test(boo
>  	int ret, i, status;
>  	struct sched_param param = {};
>  	pid_t pids[num_pre_forks];
> +	int pipe_fds[2];
> +
> +	SCX_FAIL_IF(pipe(pipe_fds) < 0, "Failed to create pipe");
> 
>  	skel = init_enable_count__open();
>  	SCX_FAIL_IF(!skel, "Failed to open");
> @@ -38,26 +42,34 @@ static enum scx_test_status run_test(boo
>  	 * ensure (at least in practical terms) that there are more tasks that
>  	 * transition from SCHED_OTHER -> SCHED_EXT than there are tasks that
>  	 * take the fork() path either below or in other processes.
> +	 *
> +	 * All children will block on read() on the pipe until the parent closes
> +	 * the write end after attaching the scheduler, which signals all of
> +	 * them to exit simultaneously. Auto-reap so we don't have to wait on
> +	 * them.
>  	 */
> +	signal(SIGCHLD, SIG_IGN);
>  	for (i = 0; i < num_pre_forks; i++) {
> -		pids[i] = fork();
> -		SCX_FAIL_IF(pids[i] < 0, "Failed to fork child");
> -		if (pids[i] == 0) {
> -			sleep(1);
> +		pid_t pid = fork();
> +
> +		SCX_FAIL_IF(pid < 0, "Failed to fork child");
> +		if (pid == 0) {
> +			char buf;
> +
> +			close(pipe_fds[1]);
> +			read(pipe_fds[0], &buf, 1);
> +			close(pipe_fds[0]);
>  			exit(0);
>  		}
>  	}
> +	close(pipe_fds[0]);
> 
>  	link = bpf_map__attach_struct_ops(skel->maps.init_enable_count_ops);
>  	SCX_FAIL_IF(!link, "Failed to attach struct_ops");
> 
> -	for (i = 0; i < num_pre_forks; i++) {
> -		SCX_FAIL_IF(waitpid(pids[i], &status, 0) != pids[i],
> -			    "Failed to wait for pre-forked child\n");
> -
> -		SCX_FAIL_IF(status != 0, "Pre-forked child %d exited with status %d\n", i,
> -			    status);
> -	}
> +	/* Signal all pre-forked children to exit. */
> +	close(pipe_fds[1]);
> +	signal(SIGCHLD, SIG_DFL);
> 
>  	bpf_link__destroy(link);
>  	SCX_GE(skel->bss->init_task_cnt, num_pre_forks);