selftests/sched_ext: Add test for sched_ext dl_server

Add a selftest to validate the correct behavior of the deadline server
for the ext_sched_class.

Co-developed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Tested-by: Christian Loehle <christian.loehle@arm.com>
Link: https://patch.msgid.link/20260126100050.3854740-7-arighi@nvidia.com

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)))

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",
+};

tools/testing/selftests/sched_ext/rt_stall.c

@@ -0,0 +1,240 @@
+// 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 <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(bool is_ext)
+{
+	/*
+	 * 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% */
+	const char *class_str = is_ext ? "EXT" : "FAIR";
+
+	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 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 %s task (PID %d)\n",
+				   class_str, ext_pid);
+	ksft_print_msg("Runtime of %s task (PID %d) is %f seconds\n",
+		       class_str, 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("%s task got %.2f%% of total runtime\n",
+		       class_str, actual_ratio * 100);
+
+	if (actual_ratio >= expected_min_ratio) {
+		ksft_test_result_pass("PASS: %s task got more than %.2f%% of runtime\n",
+				      class_str, expected_min_ratio * 100);
+		return true;
+	}
+	ksft_test_result_fail("FAIL: %s task got less than %.2f%% of runtime\n",
+			      class_str, expected_min_ratio * 100);
+	return false;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+	struct rt_stall *skel = ctx;
+	struct bpf_link *link = NULL;
+	bool res;
+	int i;
+
+	/*
+	 * Test if the dl_server is working both with and without the
+	 * sched_ext scheduler attached.
+	 *
+	 * This ensures all the scenarios are covered:
+	 *   - fair_server stop -> ext_server start
+	 *   - ext_server stop -> fair_server stop
+	 */
+	for (i = 0; i < 4; i++) {
+		bool is_ext = i % 2;
+
+		if (is_ext) {
+			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(is_ext);
+		if (is_ext) {
+			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)