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selftests/sched_ext: Fix rt_stall flaky failure

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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: be621a7634 ("selftests/sched_ext: Add test for sched_ext dl_server")
Assisted-by: claude-opus-4-6-v1
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Ihor Solodrai 2026-02-13 10:21:36 -08:00 • committed by Tejun Heo
parent 048714d9df
commit 0b82cc331d
1 changed files with 49 additions and 0 deletions
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49
tools/testing/selftests/sched_ext/rt_stall.c
View file

@ -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);