Commit 0b82cc33 authored by Ihor Solodrai's avatar Ihor Solodrai Committed by Tejun Heo
Browse files

selftests/sched_ext: Fix rt_stall flaky failure 

The rt_stall test measures the runtime ratio between an EXT and an RT
task pinned to the same CPU, verifying that the deadline server prevents
RT tasks from starving SCHED_EXT tasks. It expects the EXT task to get
at least 4% of CPU time.

The test is flaky because sched_stress_test() calls sleep(RUN_TIME)
immediately after fork(), without waiting for the RT child to complete
its setup (set_affinity + set_sched). If the RT child experiences
scheduling latency before completing setup, that delay eats into the
measurement window: the RT child runs for less than RUN_TIME seconds,
and the EXT task's measured ratio drops below the 4% threshold.

For example, in the failing CI run [1]:
  EXT=0.140s RT=4.750s total=4.890s (expected ~5.0s)
  ratio=2.86% < 4% → FAIL

The 110ms gap (5.0 - 4.89) corresponds to the RT child's setup time
being counted inside the measurement window, during which fewer
deadline server ticks fire for the EXT task.

Fix by using pipes to synchronize: each child signals the parent after
completing its setup, and the parent waits for both signals before
starting sleep(RUN_TIME). This ensures the measurement window only
counts time when both tasks are fully configured and competing.

[1] https://github.com/kernel-patches/bpf/actions/runs/21961895809/job/63442490449



Fixes: be621a76 ("selftests/sched_ext: Add test for sched_ext dl_server")
Assisted-by: claude-opus-4-6-v1
Signed-off-by: default avatarIhor Solodrai <ihor.solodrai@linux.dev>
Reviewed-by: default avatarAndrea Righi <arighi@nvidia.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent 048714d9

tools/testing/selftests/sched_ext/rt_stall.c

+49 −0
Original line number Diff line number Diff line
@@ -23,6 +23,30 @@ 
#define CORE_ID		0	/* CPU to pin tasks to */

#define RUN_TIME        5	/* How long to run the test in seconds */



/* Signal the parent that setup is complete by writing to a pipe */

static void signal_ready(int fd)

{

	char c = 1;



	if (write(fd, &c, 1) != 1) {

		perror("write to ready pipe");

		exit(EXIT_FAILURE);

	}

	close(fd);

}



/* Wait for a child to signal readiness via a pipe */

static void wait_ready(int fd)

{

	char c;



	if (read(fd, &c, 1) != 1) {

		perror("read from ready pipe");

		exit(EXIT_FAILURE);

	}

	close(fd);

}



/* Simple busy-wait function for test tasks */

static void process_func(void)

{
@@ -122,14 +146,24 @@ static bool sched_stress_test(bool is_ext) 


	float ext_runtime, rt_runtime, actual_ratio;

	int ext_pid, rt_pid;

	int ext_ready[2], rt_ready[2];



	ksft_print_header();

	ksft_set_plan(1);



	if (pipe(ext_ready) || pipe(rt_ready)) {

		perror("pipe");

		ksft_exit_fail();

	}



	/* Create and set up a EXT task */

	ext_pid = fork();

	if (ext_pid == 0) {

		close(ext_ready[0]);

		close(rt_ready[0]);

		close(rt_ready[1]);

		set_affinity(CORE_ID);

		signal_ready(ext_ready[1]);

		process_func();

		exit(0);

	} else if (ext_pid < 0) {
@@ -140,8 +174,12 @@ static bool sched_stress_test(bool is_ext) 
	/* Create an RT task */

	rt_pid = fork();

	if (rt_pid == 0) {

		close(ext_ready[0]);

		close(ext_ready[1]);

		close(rt_ready[0]);

		set_affinity(CORE_ID);

		set_sched(SCHED_FIFO, 50);

		signal_ready(rt_ready[1]);

		process_func();

		exit(0);

	} else if (rt_pid < 0) {
@@ -149,6 +187,17 @@ static bool sched_stress_test(bool is_ext) 
		ksft_exit_fail();

	}



	/*

	 * Wait for both children to complete their setup (affinity and

	 * scheduling policy) before starting the measurement window.

	 * This prevents flaky failures caused by the RT child's setup

	 * time eating into the measurement period.

	 */

	close(ext_ready[1]);

	close(rt_ready[1]);

	wait_ready(ext_ready[0]);

	wait_ready(rt_ready[0]);



	/* Let the processes run for the specified time */

	sleep(RUN_TIME);