Re: [PATCH 10/11] selftests/sched_ext: Add test for sched_ext dl_server

Christian Loehle
2025-10-30T16:49:54Z

On 10/29/25 19:08, Andrea Righi wrote:
> Add a selftest to validate the correct behavior of the deadline server
> for the ext_sched_class.
>
> v3: - add a comment to explain the 4% threshold (Emil Tsalapatis)
> v2: - replaced occurences of CFS in the test with EXT (Joel Fernandes)
>
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
> tools/testing/selftests/sched_ext/Makefile | 1 +
> .../selftests/sched_ext/rt_stall.bpf.c | 23 ++
> tools/testing/selftests/sched_ext/rt_stall.c | 222 ++++++++++++++++++
> 3 files changed, 246 insertions(+)
> create mode 100644 tools/testing/selftests/sched_ext/rt_stall.bpf.c
> create mode 100644 tools/testing/selftests/sched_ext/rt_stall.c
>
> diff --git a/tools/testing/selftests/sched_ext/Makefile b/tools/testing/selftests/sched_ext/Makefile
> index 5fe45f9c5f8fd..c9255d1499b6e 100644
> --- a/tools/testing/selftests/sched_ext/Makefile
> +++ b/tools/testing/selftests/sched_ext/Makefile
> @@ -183,6 +183,7 @@ auto-test-targets := \
> select_cpu_dispatch_bad_dsq \
> select_cpu_dispatch_dbl_dsp \
> select_cpu_vtime \
> + rt_stall \
> test_example \
>
> testcase-targets := $(addsuffix .o,$(addprefix $(SCXOBJ_DIR)/,$(auto-test-targets)))
> diff --git a/tools/testing/selftests/sched_ext/rt_stall.bpf.c b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
> new file mode 100644
> index 0000000000000..80086779dd1eb
> --- /dev/null
> +++ b/tools/testing/selftests/sched_ext/rt_stall.bpf.c
> @@ -0,0 +1,23 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * A scheduler that verified if RT tasks can stall SCHED_EXT tasks.
> + *
> + * Copyright (c) 2025 NVIDIA Corporation.
> + */
> +
> +#include <scx/common.bpf.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +UEI_DEFINE(uei);
> +
> +void BPF_STRUCT_OPS(rt_stall_exit, struct scx_exit_info *ei)
> +{
> + UEI_RECORD(uei, ei);
> +}
> +
> +SEC(".struct_ops.link")
> +struct sched_ext_ops rt_stall_ops = {
> + .exit = (void *)rt_stall_exit,
> + .name = "rt_stall",
> +};
> diff --git a/tools/testing/selftests/sched_ext/rt_stall.c b/tools/testing/selftests/sched_ext/rt_stall.c
> new file mode 100644
> index 0000000000000..d0ffa0e72b37b
> --- /dev/null
> +++ b/tools/testing/selftests/sched_ext/rt_stall.c
> @@ -0,0 +1,222 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2025 NVIDIA Corporation.
> + */
> +#define _GNU_SOURCE
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <sched.h>
> +#include <sys/prctl.h>
> +#include <sys/types.h>
> +#include <sys/wait.h>
> +#include <time.h>
> +#include <linux/sched.h>
> +#include <signal.h>
> +#include <bpf/bpf.h>
> +#include <scx/common.h>
> +#include <sys/wait.h>
> +#include <unistd.h>
> +#include "rt_stall.bpf.skel.h"
> +#include "scx_test.h"
> +#include "../kselftest.h"
> +
> +#define CORE_ID 0 /* CPU to pin tasks to */
> +#define RUN_TIME 5 /* How long to run the test in seconds */
> +
> +/* Simple busy-wait function for test tasks */
> +static void process_func(void)
> +{
> + while (1) {
> + /* Busy wait */
> + for (volatile unsigned long i = 0; i < 10000000UL; i++)
> + ;
> + }
> +}
> +
> +/* Set CPU affinity to a specific core */
> +static void set_affinity(int cpu)
> +{
> + cpu_set_t mask;
> +
> + CPU_ZERO(&mask);
> + CPU_SET(cpu, &mask);
> + if (sched_setaffinity(0, sizeof(mask), &mask) != 0) {
> + perror("sched_setaffinity");
> + exit(EXIT_FAILURE);
> + }
> +}
> +
> +/* Set task scheduling policy and priority */
> +static void set_sched(int policy, int priority)
> +{
> + struct sched_param param;
> +
> + param.sched_priority = priority;
> + if (sched_setscheduler(0, policy, &param) != 0) {
> + perror("sched_setscheduler");
> + exit(EXIT_FAILURE);
> + }
> +}
> +
> +/* Get process runtime from /proc/<pid>/stat */
> +static float get_process_runtime(int pid)
> +{
> + char path[256];
> + FILE *file;
> + long utime, stime;
> + int fields;
> +
> + snprintf(path, sizeof(path), "/proc/%d/stat", pid);
> + file = fopen(path, "r");
> + if (file == NULL) {
> + perror("Failed to open stat file");
> + return -1;
> + }
> +
> + /* Skip the first 13 fields and read the 14th and 15th */
> + fields = fscanf(file,
> + "%*d %*s %*c %*d %*d %*d %*d %*d %*u %*u %*u %*u %*u %lu %lu",
> + &utime, &stime);
> + fclose(file);
> +
> + if (fields != 2) {
> + fprintf(stderr, "Failed to read stat file\n");
> + return -1;
> + }
> +
> + /* Calculate the total time spent in the process */
> + long total_time = utime + stime;
> + long ticks_per_second = sysconf(_SC_CLK_TCK);
> + float runtime_seconds = total_time * 1.0 / ticks_per_second;
> +
> + return runtime_seconds;
> +}
> +
> +static enum scx_test_status setup(void **ctx)
> +{
> + struct rt_stall *skel;
> +
> + skel = rt_stall__open();
> + SCX_FAIL_IF(!skel, "Failed to open");
> + SCX_ENUM_INIT(skel);
> + SCX_FAIL_IF(rt_stall__load(skel), "Failed to load skel");
> +
> + *ctx = skel;
> +
> + return SCX_TEST_PASS;
> +}
> +
> +static bool sched_stress_test(void)
> +{
> + /*
> + * We're expecting the EXT task to get around 5% of CPU time when
> + * competing with the RT task (small 1% fluctuations are expected).
> + *
> + * However, the EXT task should get at least 4% of the CPU to prove
> + * that the EXT deadline server is working correctly. A percentage
> + * less than 4% indicates a bug where RT tasks can potentially
> + * stall SCHED_EXT tasks, causing the test to fail.
> + */
> + const float expected_min_ratio = 0.04; /* 4% */
> +
> + float ext_runtime, rt_runtime, actual_ratio;
> + int ext_pid, rt_pid;
> +
> + ksft_print_header();
> + ksft_set_plan(1);
> +
> + /* Create and set up a EXT task */
> + ext_pid = fork();
> + if (ext_pid == 0) {
> + set_affinity(CORE_ID);
> + process_func();
> + exit(0);
> + } else if (ext_pid < 0) {
> + perror("fork for EXT task");
> + ksft_exit_fail();
> + }
> +
> + /* Create an RT task */
> + rt_pid = fork();
> + if (rt_pid == 0) {
> + set_affinity(CORE_ID);
> + set_sched(SCHED_FIFO, 50);
> + process_func();
> + exit(0);
> + } else if (rt_pid < 0) {
> + perror("fork for RT task");
> + ksft_exit_fail();
> + }
> +
> + /* Let the processes run for the specified time */
> + sleep(RUN_TIME);
> +
> + /* Get runtime for the EXT task */
> + ext_runtime = get_process_runtime(ext_pid);
> + if (ext_runtime == -1)
> + ksft_exit_fail_msg("Error getting runtime for EXT task (PID %d)\n", ext_pid);
> + ksft_print_msg("Runtime of EXT task (PID %d) is %f seconds\n",
> + ext_pid, ext_runtime);
> +
> + /* Get runtime for the RT task */
> + rt_runtime = get_process_runtime(rt_pid);
> + if (rt_runtime == -1)
> + ksft_exit_fail_msg("Error getting runtime for RT task (PID %d)\n", rt_pid);
> + ksft_print_msg("Runtime of RT task (PID %d) is %f seconds\n", rt_pid, rt_runtime);
> +
> + /* Kill the processes */
> + kill(ext_pid, SIGKILL);
> + kill(rt_pid, SIGKILL);
> + waitpid(ext_pid, NULL, 0);
> + waitpid(rt_pid, NULL, 0);
> +
> + /* Verify that the scx task got enough runtime */
> + actual_ratio = ext_runtime / (ext_runtime + rt_runtime);
> + ksft_print_msg("EXT task got %.2f%% of total runtime\n", actual_ratio * 100);
> +
> + if (actual_ratio >= expected_min_ratio) {
> + ksft_test_result_pass("PASS: EXT task got more than %.2f%% of runtime\n",
> + expected_min_ratio * 100);
> + return true;
> + }
> + ksft_test_result_fail("FAIL: EXT task got less than %.2f%% of runtime\n",
> + expected_min_ratio * 100);
> + return false;
> +}
> +
> +static enum scx_test_status run(void *ctx)
> +{
> + struct rt_stall *skel = ctx;
> + struct bpf_link *link;
> + bool res;
> +
> + link = bpf_map__attach_struct_ops(skel->maps.rt_stall_ops);
> + SCX_FAIL_IF(!link, "Failed to attach scheduler");
> +
> + res = sched_stress_test();
> +
> + SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
> + bpf_link__destroy(link);
> +
> + if (!res)
> + ksft_exit_fail();
> +
> + return SCX_TEST_PASS;
> +}
> +
> +static void cleanup(void *ctx)
> +{
> + struct rt_stall *skel = ctx;
> +
> + rt_stall__destroy(skel);
> +}
> +
> +struct scx_test rt_stall = {
> + .name = "rt_stall",
> + .description = "Verify that RT tasks cannot stall SCHED_EXT tasks",
> + .setup = setup,
> + .run = run,
> + .cleanup = cleanup,
> +};
> +REGISTER_SCX_TEST(&rt_stall)

I'd still prefer something like the below to also test if the
fair_server stop -> ext_server start -> fair_server start -> ext_server stop
flow works correctly, but FWIW
Tested-by: Christian Loehle <christian.loehle@arm.com>

------8<------
@@ -188,19 +188,24 @@ static bool sched_stress_test(void)
static enum scx_test_status run(void *ctx)
{
struct rt_stall *skel = ctx;
- struct bpf_link *link;
+ struct bpf_link *link = NULL;
bool res;

- link = bpf_map__attach_struct_ops(skel->maps.rt_stall_ops);
- SCX_FAIL_IF(!link, "Failed to attach scheduler");
-
- res = sched_stress_test();
-
- SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
- bpf_link__destroy(link);
-
- if (!res)
- ksft_exit_fail();
+ for (int i = 0; i < 4; i++) {
+ if (i % 2) {
+ memset(&skel->data->uei, 0, sizeof(skel->data->uei));
+ link = bpf_map__attach_struct_ops(skel->maps.rt_stall_ops);
+ SCX_FAIL_IF(!link, "Failed to attach scheduler");
+ }
+ res = sched_stress_test();
+ if (i % 2) {
+ SCX_EQ(skel->data->uei.kind, EXIT_KIND(SCX_EXIT_NONE));
+ bpf_link__destroy(link);
+ }
+
+ if (!res)
+ ksft_exit_fail();
+ }

return SCX_TEST_PASS;
}