scx: Add selftests for hotplug operations

We've recently added some logic related to hotplug:

- If a hotplug event occurs and a scheduler hasn't implemented a
  callback for it, we automatically exit the scheduler with specific,
  built-in exit codes

- With scx_bpf_exit(), a scheduler can choose to manually exit the
  scheduler in a hotplug event, or do something else. In any case, the
  scheduler should _not_ be automatically exited by the kernel

Let's add selftests to validate these conditions.

Signed-off-by: David Vernet <[email protected]>

tools/testing/selftests/sched_ext/Makefile

@@ -166,6 +166,7 @@ auto-test-targets :=			\
 	ddsp_bogus_dsq_fail		\
 	ddsp_vtimelocal_fail		\
 	exit				\
+	hotplug				\
 	init_enable_count		\
 	maximal				\
 	maybe_null			\
@@ -195,7 +196,10 @@ $(testcase-targets): $(SCXOBJ_DIR)/%.o: %.c $(SCXOBJ_DIR)/runner.o $(all_test_bp
 	$(eval test=$(patsubst %.o,%.c,$(notdir $@)))
 	$(CC) $(CFLAGS) -c $< -o $@ $(SCXOBJ_DIR)/runner.o
 
-runner: $(SCXOBJ_DIR)/runner.o $(BPFOBJ) $(testcase-targets)
+$(SCXOBJ_DIR)/util.o: util.c | $(SCXOBJ_DIR)
+	$(CC) $(CFLAGS) -c $< -o $@
+
+runner: $(SCXOBJ_DIR)/runner.o $(SCXOBJ_DIR)/util.o $(BPFOBJ) $(testcase-targets)
 	@echo "$(testcase-targets)"
 	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
 

tools/testing/selftests/sched_ext/hotplug.bpf.c

@@ -0,0 +1,56 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 David Vernet <[email protected]>
+ */
+
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+#include "hotplug_test.h"
+
+const volatile int hotplug_point;
+UEI_DEFINE(uei);
+
+void BPF_STRUCT_OPS(hotplug_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+static void exit_from_hotplug(s32 cpu, bool onlining)
+{
+	s64 code = SCX_ECODE_ACT_RESTART | HOTPLUG_EXIT_RSN;
+
+	if (onlining)
+		code |= HOTPLUG_ONLINING;
+
+	scx_bpf_exit(code, "hotplug event detected (%d going %s)", cpu,
+		     onlining ? "online" : "offline");
+}
+
+void BPF_STRUCT_OPS(hotplug_cpu_online, s32 cpu)
+{
+	exit_from_hotplug(cpu, true);
+}
+
+void BPF_STRUCT_OPS(hotplug_cpu_offline, s32 cpu)
+{
+	exit_from_hotplug(cpu, false);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops hotplug_cb_ops = {
+	.cpu_online		= hotplug_cpu_online,
+	.cpu_offline		= hotplug_cpu_offline,
+	.exit			= hotplug_exit,
+	.name			= "hotplug_cbs",
+	.timeout_ms		= 1000U,
+};
+
+SEC(".struct_ops.link")
+struct sched_ext_ops hotplug_nocb_ops = {
+	.exit			= hotplug_exit,
+	.name			= "hotplug_nocbs",
+	.timeout_ms		= 1000U,
+};

tools/testing/selftests/sched_ext/hotplug.c

@@ -0,0 +1,134 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 David Vernet <[email protected]>
+ */
+#include <bpf/bpf.h>
+#include <sched.h>
+#include <scx/common.h>
+#include <sched.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "hotplug_test.h"
+#include "hotplug.bpf.skel.h"
+#include "minimal_uei.bpf.skel.h"
+#include "scx_test.h"
+#include "util.h"
+
+struct hotplug *skel;
+
+const char *online_path = "/sys/devices/system/cpu/cpu1/online";
+
+static bool is_cpu_online(void)
+{
+	return file_read_long(online_path) > 0;
+}
+
+static void toggle_online_status(bool online)
+{
+	long val = online ? 1 : 0;
+	int ret;
+
+	ret = file_write_long(online_path, val);
+	if (ret != 0)
+		fprintf(stderr, "Failed to bring CPU %s (%s)",
+			online ? "online" : "offline", strerror(errno));
+}
+
+static enum scx_test_status setup(void **ctx)
+{
+	if (!is_cpu_online())
+		return SCX_TEST_SKIP;
+
+	skel = hotplug__open_and_load();
+	if (!skel) {
+		SCX_ERR("Failed to open and load hotplug skel");
+		return SCX_TEST_FAIL;
+	}
+
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status test_hotplug(bool onlining, bool cbs_defined)
+{
+	struct bpf_link *link;
+	long kind, code;
+
+	SCX_ASSERT(is_cpu_online());
+
+	/* Testing the offline -> online path, so go offline before starting */
+	if (onlining)
+		toggle_online_status(0);
+
+	if (cbs_defined) {
+		kind = SCX_KIND_VAL(SCX_EXIT_UNREG_BPF);
+		code = SCX_ECODE_VAL(SCX_ECODE_ACT_RESTART) | HOTPLUG_EXIT_RSN;
+		if (onlining)
+			code |= HOTPLUG_ONLINING;
+	} else {
+		kind = SCX_KIND_VAL(SCX_EXIT_UNREG_KERN);
+		code = SCX_ECODE_VAL(SCX_ECODE_ACT_RESTART) |
+		       SCX_ECODE_VAL(SCX_ECODE_RSN_HOTPLUG);
+	}
+
+	if (cbs_defined)
+		link = bpf_map__attach_struct_ops(skel->maps.hotplug_cb_ops);
+	else
+		link = bpf_map__attach_struct_ops(skel->maps.hotplug_nocb_ops);
+
+	if (!link) {
+		SCX_ERR("Failed to attach scheduler");
+		return SCX_TEST_FAIL;
+	}
+
+	toggle_online_status(onlining ? 1 : 0);
+
+	while (!UEI_EXITED(skel, uei))
+		sched_yield();
+
+	SCX_EQ(UEI_KIND(skel, uei), kind);
+	SCX_EQ(UEI_ECODE(skel, uei), code);
+
+	if (!onlining)
+		toggle_online_status(1);
+
+	bpf_link__destroy(link);
+
+	UEI_RESET(skel, uei);
+
+	return SCX_TEST_PASS;
+}
+
+static enum scx_test_status run(void *ctx)
+{
+
+#define HP_TEST(__onlining, __cbs_defined) ({				\
+	if (test_hotplug(__onlining, __cbs_defined) != SCX_TEST_PASS)	\
+		return SCX_TEST_FAIL;					\
+})
+
+	HP_TEST(true, true);
+	HP_TEST(false, true);
+	HP_TEST(true, false);
+	HP_TEST(false, false);
+
+#undef HP_TEST
+
+	return SCX_TEST_PASS;
+}
+
+static void cleanup(void *ctx)
+{
+	hotplug__destroy(skel);
+	toggle_online_status(1);
+}
+
+struct scx_test hotplug_test = {
+	.name = "hotplug",
+	.description = "Verify hotplug behavior",
+	.setup = setup,
+	.run = run,
+	.cleanup = cleanup,
+};
+REGISTER_SCX_TEST(&hotplug_test)

tools/testing/selftests/sched_ext/hotplug_test.h

@@ -0,0 +1,15 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 David Vernet <[email protected]>
+ */
+
+#ifndef __HOTPLUG_TEST_H__
+#define __HOTPLUG_TEST_H__
+
+enum hotplug_test_flags {
+	HOTPLUG_EXIT_RSN = 1LLU << 0,
+	HOTPLUG_ONLINING = 1LLU << 1,
+};
+
+#endif  // # __HOTPLUG_TEST_H__

tools/testing/selftests/sched_ext/minimal_uei.bpf.c

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * A completely hotplug_restart scheduler.
+ *
+ * This scheduler defines a minimal scheduler that defines a UEI, but nothing
+ * else.
+ *
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 David Vernet <[email protected]>
+ */
+
+#include <scx/common.bpf.h>
+
+char _license[] SEC("license") = "GPL";
+
+UEI_DEFINE(uei);
+
+void BPF_STRUCT_OPS(minimal_uei_exit, struct scx_exit_info *ei)
+{
+	UEI_RECORD(uei, ei);
+}
+
+SEC(".struct_ops.link")
+struct sched_ext_ops minimal_uei_ops = {
+	.exit			= minimal_uei_exit,
+	.name			= "minimal_uei",
+};

tools/testing/selftests/sched_ext/scx_test.h

@@ -110,4 +110,22 @@ void scx_test_register(struct scx_test *test);
 #define SCX_ASSERT(_x) SCX_FAIL_IF(!(_x), "Expected %s to be true (%lu)",		\
 				   #_x, (u64)(_x))
 
+#define SCX_ECODE_VAL(__ecode) ({						\
+        u64 __val = 0;								\
+	bool __found = false;							\
+										\
+	__found = __COMPAT_read_enum("scx_exit_code", #__ecode, &__val);	\
+	SCX_ASSERT(__found);							\
+	(s64)__val;								\
+})
+
+#define SCX_KIND_VAL(__kind) ({							\
+        u64 __val = 0;								\
+	bool __found = false;							\
+										\
+	__found = __COMPAT_read_enum("scx_exit_kind", #__kind, &__val);		\
+	SCX_ASSERT(__found);							\
+	__val;									\
+})
+
 #endif  // # __SCX_TEST_H__

tools/testing/selftests/sched_ext/util.c

@@ -0,0 +1,71 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 David Vernet <[email protected]>
+ */
+#include <errno.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+/* Returns read len on success, or -errno on failure. */
+static ssize_t read_text(const char *path, char *buf, size_t max_len)
+{
+	ssize_t len;
+	int fd;
+
+	fd = open(path, O_RDONLY);
+	if (fd < 0)
+		return -errno;
+
+	len = read(fd, buf, max_len - 1);
+
+	if (len >= 0)
+		buf[len] = 0;
+
+	close(fd);
+	return len < 0 ? -errno : len;
+}
+
+/* Returns written len on success, or -errno on failure. */
+static ssize_t write_text(const char *path, char *buf, ssize_t len)
+{
+	int fd;
+	ssize_t written;
+
+	fd = open(path, O_WRONLY | O_APPEND);
+	if (fd < 0)
+		return -errno;
+
+	written = write(fd, buf, len);
+	close(fd);
+	return written < 0 ? -errno : written;
+}
+
+long file_read_long(const char *path)
+{
+	char buf[128];
+
+
+	if (read_text(path, buf, sizeof(buf)) <= 0)
+		return -1;
+
+	return atol(buf);
+}
+
+int file_write_long(const char *path, long val)
+{
+	char buf[64];
+	int ret;
+
+	ret = sprintf(buf, "%lu", val);
+	if (ret < 0)
+		return ret;
+
+	if (write_text(path, buf, sizeof(buf)) <= 0)
+		return -1;
+
+	return 0;
+}

tools/testing/selftests/sched_ext/util.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2024 Meta Platforms, Inc. and affiliates.
+ * Copyright (c) 2024 Tejun Heo <[email protected]>
+ */
+
+#ifndef __SCX_TEST_UTIL_H__
+#define __SCX_TEST_UTIL_H__
+
+long file_read_long(const char *path);
+int file_write_long(const char *path, long val);
+
+#endif // __SCX_TEST_H__