[PATCH v8 -tip 23/26] kselftest: Add tests for core-sched interface

[Joel Fernandes (Google)]

Tested-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 tools/testing/selftests/sched/.gitignore      |   1 +
 tools/testing/selftests/sched/Makefile        |  14 +
 tools/testing/selftests/sched/config          |   1 +
 .../testing/selftests/sched/test_coresched.c  | 840 ++++++++++++++++++
 4 files changed, 856 insertions(+)
 create mode 100644 tools/testing/selftests/sched/.gitignore
 create mode 100644 tools/testing/selftests/sched/Makefile
 create mode 100644 tools/testing/selftests/sched/config
 create mode 100644 tools/testing/selftests/sched/test_coresched.c

diff --git a/tools/testing/selftests/sched/.gitignore b/tools/testing/selftests/sched/.gitignore
new file mode 100644
index 000000000000..4660929b0b9a
--- /dev/null
+++ b/tools/testing/selftests/sched/.gitignore
@@ -0,0 +1 @@
+test_coresched
diff --git a/tools/testing/selftests/sched/Makefile b/tools/testing/selftests/sched/Makefile
new file mode 100644
index 000000000000..e43b74fc5d7e
--- /dev/null
+++ b/tools/testing/selftests/sched/Makefile
@@ -0,0 +1,14 @@
+# SPDX-License-Identifier: GPL-2.0+
+
+ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
+CLANG_FLAGS += -no-integrated-as
+endif
+
+CFLAGS += -O2 -Wall -g -I./ -I../../../../usr/include/  -Wl,-rpath=./ \
+	  $(CLANG_FLAGS)
+LDLIBS += -lpthread
+
+TEST_GEN_FILES := test_coresched
+TEST_PROGS := test_coresched
+
+include ../lib.mk
diff --git a/tools/testing/selftests/sched/config b/tools/testing/selftests/sched/config
new file mode 100644
index 000000000000..e8b09aa7c0c4
--- /dev/null
+++ b/tools/testing/selftests/sched/config
@@ -0,0 +1 @@
+CONFIG_SCHED_DEBUG=y
diff --git a/tools/testing/selftests/sched/test_coresched.c b/tools/testing/selftests/sched/test_coresched.c
new file mode 100644
index 000000000000..2fdefb843115
--- /dev/null
+++ b/tools/testing/selftests/sched/test_coresched.c
@@ -0,0 +1,840 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Core-scheduling selftests.
+ *
+ * Copyright (C) 2020, Joel Fernandes.
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/mount.h>
+#include <sys/prctl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <time.h>
+#include <unistd.h>
+
+#ifndef PR_SCHED_CORE_SHARE
+#define PR_SCHED_CORE_SHARE 59
+#endif
+
+#ifndef DEBUG_PRINT
+#define dprint(...)
+#else
+#define dprint(str, args...) printf("DEBUG: %s: " str "\n", __func__, ##args)
+#endif
+
+void print_banner(char *s)
+{
+    printf("coresched: %s:  ", s);
+}
+
+void print_pass(void)
+{
+    printf("PASS\n");
+}
+
+void assert_cond(int cond, char *str)
+{
+    if (!cond) {
+	printf("Error: %s\n", str);
+	abort();
+    }
+}
+
+char *make_group_root(void)
+{
+	char *mntpath, *mnt;
+	int ret;
+
+	mntpath = malloc(50);
+	if (!mntpath) {
+	    perror("Failed to allocate mntpath\n");
+	    abort();
+	}
+
+	sprintf(mntpath, "/tmp/coresched-test-XXXXXX");
+	mnt = mkdtemp(mntpath);
+	if (!mnt) {
+		perror("Failed to create mount: ");
+		exit(-1);
+	}
+
+	ret = mount("nodev", mnt, "cgroup", 0, "cpu");
+	if (ret == -1) {
+		perror("Failed to mount cgroup: ");
+		exit(-1);
+	}
+
+	return mnt;
+}
+
+char *read_group_cookie(char *cgroup_path)
+{
+    char path[50] = {}, *val;
+    int fd;
+
+    sprintf(path, "%s/cpu.core_group_cookie", cgroup_path);
+    fd = open(path, O_RDONLY, 0666);
+    if (fd == -1) {
+	perror("Open of cgroup tag path failed: ");
+	abort();
+    }
+
+    val = calloc(1, 50);
+    if (read(fd, val, 50) == -1) {
+	perror("Failed to read group cookie: ");
+	abort();
+    }
+
+    val[strcspn(val, "\r\n")] = 0;
+
+    close(fd);
+    return val;
+}
+
+void assert_group_tag(char *cgroup_path, char *tag)
+{
+    char tag_path[50] = {}, rdbuf[8] = {};
+    int tfd;
+
+    sprintf(tag_path, "%s/cpu.core_tag", cgroup_path);
+    tfd = open(tag_path, O_RDONLY, 0666);
+    if (tfd == -1) {
+	perror("Open of cgroup tag path failed: ");
+	abort();
+    }
+
+    if (read(tfd, rdbuf, 1) != 1) {
+	perror("Failed to enable coresched on cgroup: ");
+	abort();
+    }
+
+    if (strcmp(rdbuf, tag)) {
+	printf("Group tag does not match (exp: %s, act: %s)\n", tag, rdbuf);
+	abort();
+    }
+
+    if (close(tfd) == -1) {
+	perror("Failed to close tag fd: ");
+	abort();
+    }
+}
+
+void assert_group_color(char *cgroup_path, const char *color)
+{
+    char tag_path[50] = {}, rdbuf[8] = {};
+    int tfd;
+
+    sprintf(tag_path, "%s/cpu.core_tag_color", cgroup_path);
+    tfd = open(tag_path, O_RDONLY, 0666);
+    if (tfd == -1) {
+	perror("Open of cgroup tag path failed: ");
+	abort();
+    }
+
+    if (read(tfd, rdbuf, 8) == -1) {
+	perror("Failed to read group color\n");
+	abort();
+    }
+
+    if (strncmp(color, rdbuf, strlen(color))) {
+	printf("Group color does not match (exp: %s, act: %s)\n", color, rdbuf);
+	abort();
+    }
+
+    if (close(tfd) == -1) {
+	perror("Failed to close color fd: ");
+	abort();
+    }
+}
+
+void color_group(char *cgroup_path, const char *color_str)
+{
+	char tag_path[50];
+	int tfd, color, ret;
+
+	color = atoi(color_str);
+
+	sprintf(tag_path, "%s/cpu.core_tag_color", cgroup_path);
+	tfd = open(tag_path, O_WRONLY, 0666);
+	if (tfd == -1) {
+		perror("Open of cgroup tag path failed: ");
+		abort();
+	}
+
+	ret = write(tfd, color_str, strlen(color_str));
+	assert_cond(color < 256 || ret == -1,
+		    "Writing invalid range color should have failed!");
+
+	if (color < 1 || color > 255) {
+	    close(tfd);
+	    return;
+	}
+
+	if (ret == -1) {
+		perror("Failed to set color on cgroup: ");
+		abort();
+	}
+
+	if (close(tfd) == -1) {
+		perror("Failed to close tag fd: ");
+		abort();
+	}
+
+	assert_group_color(cgroup_path, color_str);
+}
+
+void tag_group(char *cgroup_path)
+{
+	char tag_path[50];
+	int tfd;
+
+	sprintf(tag_path, "%s/cpu.core_tag", cgroup_path);
+	tfd = open(tag_path, O_WRONLY, 0666);
+	if (tfd == -1) {
+		perror("Open of cgroup tag path failed: ");
+		abort();
+	}
+
+	if (write(tfd, "1", 1) != 1) {
+		perror("Failed to enable coresched on cgroup: ");
+		abort();
+	}
+
+	if (close(tfd) == -1) {
+		perror("Failed to close tag fd: ");
+		abort();
+	}
+
+	assert_group_tag(cgroup_path, "1");
+}
+
+void untag_group(char *cgroup_path)
+{
+	char tag_path[50];
+	int tfd;
+
+	sprintf(tag_path, "%s/cpu.core_tag", cgroup_path);
+	tfd = open(tag_path, O_WRONLY, 0666);
+	if (tfd == -1) {
+		perror("Open of cgroup tag path failed: ");
+		abort();
+	}
+
+	if (write(tfd, "0", 1) != 1) {
+		perror("Failed to disable coresched on cgroup: ");
+		abort();
+	}
+
+	if (close(tfd) == -1) {
+		perror("Failed to close tag fd: ");
+		abort();
+	}
+
+	assert_group_tag(cgroup_path, "0");
+}
+
+char *make_group(char *parent, char *name)
+{
+	char *cgroup_path;
+	int ret;
+
+	if (!parent && !name)
+	    return make_group_root();
+
+	cgroup_path = malloc(50);
+	if (!cgroup_path) {
+	    perror("Failed to allocate cgroup_path\n");
+	    abort();
+	}
+
+	/* Make the cgroup node for this group */
+	sprintf(cgroup_path, "%s/%s", parent, name);
+	ret = mkdir(cgroup_path, 0644);
+	if (ret == -1) {
+		perror("Failed to create group in cgroup: ");
+		abort();
+	}
+
+	return cgroup_path;
+}
+
+static void del_group(char *path)
+{
+    if (rmdir(path) != 0) {
+	printf("Removal of group failed\n");
+	abort();
+    }
+
+    free(path);
+}
+
+static void del_root_group(char *path)
+{
+    if (umount(path) != 0) {
+	perror("umount of cgroup failed\n");
+	abort();
+    }
+
+    if (rmdir(path) != 0) {
+	printf("Removal of group failed\n");
+	abort();
+    }
+
+    free(path);
+}
+
+void assert_group_cookie_equal(char *c1, char *c2)
+{
+    char *v1, *v2;
+
+    v1 = read_group_cookie(c1);
+    v2 = read_group_cookie(c2);
+    if (strcmp(v1, v2)) {
+	printf("Group cookies not equal\n");
+	abort();
+    }
+
+    free(v1);
+    free(v2);
+}
+
+void assert_group_cookie_not_equal(char *c1, char *c2)
+{
+    char *v1, *v2;
+
+    v1 = read_group_cookie(c1);
+    v2 = read_group_cookie(c2);
+    if (!strcmp(v1, v2)) {
+	printf("Group cookies not equal\n");
+	abort();
+    }
+
+    free(v1);
+    free(v2);
+}
+
+void assert_group_cookie_not_zero(char *c1)
+{
+    char *v1 = read_group_cookie(c1);
+
+    v1[1] = 0;
+    if (!strcmp(v1, "0")) {
+	printf("Group cookie zero\n");
+	abort();
+    }
+    free(v1);
+}
+
+void assert_group_cookie_zero(char *c1)
+{
+    char *v1 = read_group_cookie(c1);
+
+    v1[1] = 0;
+    if (strcmp(v1, "0")) {
+	printf("Group cookie not zero");
+	abort();
+    }
+    free(v1);
+}
+
+struct task_state {
+    int pid_share;
+    char pid_str[50];
+    pthread_mutex_t m;
+    pthread_cond_t cond;
+    pthread_cond_t cond_par;
+};
+
+struct task_state *add_task(char *p)
+{
+    struct task_state *mem;
+    pthread_mutexattr_t am;
+    pthread_condattr_t a;
+    char tasks_path[50];
+    int tfd, pid, ret;
+
+    sprintf(tasks_path, "%s/tasks", p);
+    tfd = open(tasks_path, O_WRONLY, 0666);
+    if (tfd == -1) {
+	perror("Open of cgroup tasks path failed: ");
+	abort();
+    }
+
+    mem = mmap(NULL, sizeof *mem, PROT_READ | PROT_WRITE,
+	    MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+    memset(mem, 0, sizeof(*mem));
+
+    pthread_condattr_init(&a);
+    pthread_condattr_setpshared(&a, PTHREAD_PROCESS_SHARED);
+    pthread_mutexattr_init(&am);
+    pthread_mutexattr_setpshared(&am, PTHREAD_PROCESS_SHARED);
+
+    pthread_cond_init(&mem->cond, &a);
+    pthread_cond_init(&mem->cond_par, &a);
+    pthread_mutex_init(&mem->m, &am);
+
+    pid = fork();
+    if (pid == 0) {
+	while(1) {
+	    pthread_mutex_lock(&mem->m);
+	    while(!mem->pid_share)
+		pthread_cond_wait(&mem->cond, &mem->m);
+
+	    pid = mem->pid_share;
+	    mem->pid_share = 0;
+	    if (pid == -1)
+		pid = 0;
+	    prctl(PR_SCHED_CORE_SHARE, pid);
+	    pthread_mutex_unlock(&mem->m);
+	    pthread_cond_signal(&mem->cond_par);
+	}
+    }
+
+    sprintf(mem->pid_str, "%d", pid);
+    dprint("add task %d to group %s", pid, p);
+
+    ret = write(tfd, mem->pid_str, strlen(mem->pid_str));
+    assert_cond(ret != -1,
+	    "Failed to write pid into tasks");
+
+    close(tfd);
+    return mem;
+}
+
+/* Make t1 share with t2 */
+void make_tasks_share(struct task_state *t1, struct task_state *t2)
+{
+    int p2 = atoi(t2->pid_str);
+    dprint("task %s %s", t1->pid_str, t2->pid_str);
+
+    pthread_mutex_lock(&t1->m);
+    t1->pid_share = p2;
+    pthread_mutex_unlock(&t1->m);
+
+    pthread_cond_signal(&t1->cond);
+
+    pthread_mutex_lock(&t1->m);
+    while (t1->pid_share)
+	pthread_cond_wait(&t1->cond_par, &t1->m);
+    pthread_mutex_unlock(&t1->m);
+}
+
+/* Make t1 share with t2 */
+void reset_task_cookie(struct task_state *t1)
+{
+    dprint("task %s", t1->pid_str);
+
+    pthread_mutex_lock(&t1->m);
+    t1->pid_share = -1;
+    pthread_mutex_unlock(&t1->m);
+
+    pthread_cond_signal(&t1->cond);
+
+    pthread_mutex_lock(&t1->m);
+    while (t1->pid_share)
+	pthread_cond_wait(&t1->cond_par, &t1->m);
+    pthread_mutex_unlock(&t1->m);
+}
+
+char *get_task_core_cookie(char *pid)
+{
+    char proc_path[50];
+    int found = 0;
+    char *line;
+    int i, j;
+    FILE *fp;
+
+    line = malloc(1024);
+    assert_cond(!!line, "Failed to alloc memory");
+
+    sprintf(proc_path, "/proc/%s/sched", pid);
+
+    fp = fopen(proc_path, "r");
+    while ((fgets(line, 1024, fp)) != NULL)
+    {
+        if(!strstr(line, "core_cookie"))
+            continue;
+
+        for (j = 0, i = 0; i < 1024 && line[i] != '\0'; i++)
+            if (line[i] >= '0' && line[i] <= '9')
+                line[j++] = line[i];
+        line[j] = '\0';
+        found = 1;
+        break;
+    }
+
+    fclose(fp);
+
+    if (found) {
+        return line;
+    } else {
+        free(line);
+	printf("core_cookie not found. Enable SCHED_DEBUG?\n");
+	abort();
+        return NULL;
+    }
+}
+
+void assert_tasks_share(struct task_state *t1, struct task_state *t2)
+{
+    char *c1, *c2;
+
+    c1 = get_task_core_cookie(t1->pid_str);
+    c2 = get_task_core_cookie(t2->pid_str);
+    dprint("check task (%s) cookie (%s) == task (%s) cookie (%s)",
+	    t1->pid_str, c1, t2->pid_str, c2);
+    assert_cond(!strcmp(c1, c2), "Tasks don't share cookie");
+    free(c1); free(c2);
+}
+
+void assert_tasks_dont_share(struct task_state *t1,  struct task_state *t2)
+{
+    char *c1, *c2;
+    c1 = get_task_core_cookie(t1->pid_str);
+    c2 = get_task_core_cookie(t2->pid_str);
+    dprint("check task (%s) cookie (%s) != task (%s) cookie (%s)",
+	    t1->pid_str, c1, t2->pid_str, c2);
+    assert_cond(strcmp(c1, c2), "Tasks don't share cookie");
+    free(c1); free(c2);
+}
+
+void assert_group_cookie_equals_task_cookie(char *g, char *pid)
+{
+    char *gk;
+    char *tk;
+
+    gk = read_group_cookie(g);
+    tk = get_task_core_cookie(pid);
+
+    assert_cond(!strcmp(gk, tk), "Group cookie not equal to tasks'");
+
+    free(gk);
+    free(tk);
+}
+
+void assert_group_cookie_not_equals_task_cookie(char *g, char *pid)
+{
+    char *gk;
+    char *tk;
+
+    gk = read_group_cookie(g);
+    tk = get_task_core_cookie(pid);
+
+    assert_cond(strcmp(gk, tk), "Group cookie not equal to tasks'");
+
+    free(gk);
+    free(tk);
+}
+
+void kill_task(struct task_state *t)
+{
+    int pid = atoi(t->pid_str);
+
+    kill(pid, SIGKILL);
+    waitpid(pid, NULL, 0);
+}
+
+/*
+ * Test coloring. r1, y2, b3 and r4 are children of a tagged group y1. But r1,
+ * b3 and r4 are colored different from y1.  Only y2 (and thus y22) shares the
+ * same color as y1. Due to this only those have same cookie as y1. Further, r4
+ * and r1 have the same cookie as they are both colored the same.
+ *
+ *   y1 ----- r1 -- r11  (color, say red)
+ *   | \_y2 -- y22 (color, say yello (default))
+ *   \_b3 (color, say blue)
+ *   \_r4 (color, say red)
+ */
+static void test_cgroup_coloring(char *root)
+{
+    char *y1, *y2, *y22, *r1, *r11, *b3, *r4;
+
+    print_banner("TEST-CGROUP-COLORING");
+
+    y1 = make_group(root, "y1");
+    tag_group(y1);
+
+    y2 = make_group(y1, "y2");
+    y22 = make_group(y2, "y22");
+
+    r1 = make_group(y1, "y1");
+    r11 = make_group(r1, "r11");
+
+    color_group(r1, "256"); /* Wouldn't succeed. */
+    color_group(r1, "0");   /* Wouldn't succeed. */
+    color_group(r1, "254");
+
+    b3 = make_group(y1, "b3");
+    color_group(b3, "8");
+
+    r4 = make_group(y1, "r4");
+    color_group(r4, "254");
+
+    /* Check that all yellows share the same cookie. */
+    assert_group_cookie_not_zero(y1);
+    assert_group_cookie_equal(y1, y2);
+    assert_group_cookie_equal(y1, y22);
+
+    /* Check that all reds share the same cookie. */
+    assert_group_cookie_not_zero(r1);
+    assert_group_cookie_equal(r1, r11);
+    assert_group_cookie_equal(r11, r4);
+
+    /* Check that blue, red and yellow are different cookie. */
+    assert_group_cookie_not_equal(r1, b3);
+    assert_group_cookie_not_equal(b3, y1);
+
+    del_group(r11);
+    del_group(r1);
+    del_group(y22);
+    del_group(y2);
+    del_group(b3);
+    del_group(r4);
+    del_group(y1);
+    print_pass();
+}
+
+/*
+ * Test that a group's children have a cookie inherrited
+ * from their parent group _after_ the parent was tagged.
+ *
+ *   p ----- c1 - c11
+ *     \ c2 - c22
+ */
+static void test_cgroup_parent_child_tag_inherit(char *root)
+{
+    char *p, *c1, *c11, *c2, *c22;
+
+    print_banner("TEST-CGROUP-PARENT-CHILD-TAG");
+
+    p = make_group(root, "p");
+    assert_group_cookie_zero(p);
+
+    c1 = make_group(p, "c1");
+    assert_group_tag(c1, "0"); /* Child tag is "0" but inherits cookie from parent. */
+    assert_group_cookie_zero(c1);
+    assert_group_cookie_equal(c1, p);
+
+    c11 = make_group(c1, "c11");
+    assert_group_tag(c11, "0");
+    assert_group_cookie_zero(c11);
+    assert_group_cookie_equal(c11, p);
+
+    c2 = make_group(p, "c2");
+    assert_group_tag(c2, "0");
+    assert_group_cookie_zero(c2);
+    assert_group_cookie_equal(c2, p);
+
+    tag_group(p);
+
+    /* Verify c1 got the cookie */
+    assert_group_tag(c1, "0");
+    assert_group_cookie_not_zero(c1);
+    assert_group_cookie_equal(c1, p);
+
+    /* Verify c2 got the cookie */
+    assert_group_tag(c2, "0");
+    assert_group_cookie_not_zero(c2);
+    assert_group_cookie_equal(c2, p);
+
+    /* Verify c11 got the cookie */
+    assert_group_tag(c11, "0");
+    assert_group_cookie_not_zero(c11);
+    assert_group_cookie_equal(c11, p);
+
+    /*
+     * Verify c22 which is a nested group created
+     * _after_ tagging got the cookie.
+     */
+    c22 = make_group(c2, "c22");
+
+    assert_group_tag(c22, "0");
+    assert_group_cookie_not_zero(c22);
+    assert_group_cookie_equal(c22, c1);
+    assert_group_cookie_equal(c22, c11);
+    assert_group_cookie_equal(c22, c2);
+    assert_group_cookie_equal(c22, p);
+
+    del_group(c22);
+    del_group(c11);
+    del_group(c1);
+    del_group(c2);
+    del_group(p);
+    print_pass();
+}
+
+/*
+ * Test that a tagged group's children have a cookie inherrited
+ * from their parent group.
+ */
+static void test_cgroup_parent_tag_child_inherit(char *root)
+{
+    char *p, *c1, *c2, *c3;
+
+    print_banner("TEST-CGROUP-PARENT-TAG-CHILD-INHERIT");
+
+    p = make_group(root, "p");
+    assert_group_cookie_zero(p);
+    tag_group(p);
+    assert_group_cookie_not_zero(p);
+
+    c1 = make_group(p, "c1");
+    assert_group_cookie_not_zero(c1);
+    /* Child tag is "0" but it inherits cookie from parent. */
+    assert_group_tag(c1, "0");
+    assert_group_cookie_equal(c1, p);
+
+    c2 = make_group(p, "c2");
+    assert_group_tag(c2, "0");
+    assert_group_cookie_equal(c2, p);
+    assert_group_cookie_equal(c1, c2);
+
+    c3 = make_group(c1, "c3");
+    assert_group_tag(c3, "0");
+    assert_group_cookie_equal(c3, p);
+    assert_group_cookie_equal(c1, c3);
+
+    del_group(c3);
+    del_group(c1);
+    del_group(c2);
+    del_group(p);
+    print_pass();
+}
+
+static void test_prctl_in_group(char *root)
+{
+    char *p;
+    struct task_state *tsk1, *tsk2, *tsk3;
+
+    print_banner("TEST-PRCTL-IN-GROUP");
+
+    p = make_group(root, "p");
+    assert_group_cookie_zero(p);
+    tag_group(p);
+    assert_group_cookie_not_zero(p);
+
+    tsk1 = add_task(p);
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+
+    tsk2 = add_task(p);
+    assert_group_cookie_equals_task_cookie(p, tsk2->pid_str);
+
+    tsk3 = add_task(p);
+    assert_group_cookie_equals_task_cookie(p, tsk3->pid_str);
+
+    /* tsk2 share with tsk3 -- both get disconnected from CGroup. */
+    make_tasks_share(tsk2, tsk3);
+    assert_tasks_share(tsk2, tsk3);
+    assert_tasks_dont_share(tsk1, tsk2);
+    assert_tasks_dont_share(tsk1, tsk3);
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+    assert_group_cookie_not_equals_task_cookie(p, tsk2->pid_str);
+    assert_group_cookie_not_equals_task_cookie(p, tsk3->pid_str);
+
+    /* now reset tsk3 -- get connected back to CGroup. */
+    reset_task_cookie(tsk3);
+    assert_tasks_dont_share(tsk2, tsk3);
+    assert_tasks_share(tsk1, tsk3);      // tsk3 is back.
+    assert_tasks_dont_share(tsk1, tsk2); // but tsk2 is still zombie
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+    assert_group_cookie_not_equals_task_cookie(p, tsk2->pid_str);
+    assert_group_cookie_equals_task_cookie(p, tsk3->pid_str); // tsk3 is back.
+
+    /* now reset tsk2 as well to get it connected back to CGroup. */
+    reset_task_cookie(tsk2);
+    assert_tasks_share(tsk2, tsk3);
+    assert_tasks_share(tsk1, tsk3);
+    assert_tasks_share(tsk1, tsk2);
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+    assert_group_cookie_equals_task_cookie(p, tsk2->pid_str);
+    assert_group_cookie_equals_task_cookie(p, tsk3->pid_str);
+
+    /* Test the rest of the cases (2 to 4)
+     *
+     *		t1		joining		t2
+     * CASE 1:
+     * before	0				0
+     * after	new cookie			new cookie
+     *
+     * CASE 2:
+     * before	X (non-zero)			0
+     * after	0				0
+     *
+     * CASE 3:
+     * before	0				X (non-zero)
+     * after	X				X
+     *
+     * CASE 4:
+     * before	Y (non-zero)			X (non-zero)
+     * after	X				X
+     */
+
+    /* case 2: */
+    dprint("case 2");
+    make_tasks_share(tsk1, tsk1);
+    assert_tasks_dont_share(tsk1, tsk2);
+    assert_tasks_dont_share(tsk1, tsk3);
+    assert_group_cookie_not_equals_task_cookie(p, tsk1->pid_str);
+    make_tasks_share(tsk1, tsk2); /* Will reset the task cookie. */
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+
+    /* case 3: */
+    dprint("case 3");
+    assert_group_cookie_equals_task_cookie(p, tsk2->pid_str);
+    make_tasks_share(tsk2, tsk2);
+    assert_group_cookie_not_equals_task_cookie(p, tsk2->pid_str);
+    assert_tasks_dont_share(tsk2, tsk1);
+    assert_tasks_dont_share(tsk2, tsk3);
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+    make_tasks_share(tsk1, tsk2);
+    assert_group_cookie_not_equals_task_cookie(p, tsk1->pid_str);
+    assert_tasks_share(tsk1, tsk2);
+    assert_tasks_dont_share(tsk1, tsk3);
+    reset_task_cookie(tsk1);
+    reset_task_cookie(tsk2);
+
+    /* case 4: */
+    dprint("case 4");
+    assert_tasks_share(tsk1, tsk2);
+    assert_group_cookie_equals_task_cookie(p, tsk1->pid_str);
+    make_tasks_share(tsk1, tsk1);
+    assert_group_cookie_not_equals_task_cookie(p, tsk1->pid_str);
+    assert_group_cookie_equals_task_cookie(p, tsk2->pid_str);
+    make_tasks_share(tsk2, tsk2);
+    assert_group_cookie_not_equals_task_cookie(p, tsk2->pid_str);
+    assert_tasks_dont_share(tsk1, tsk2);
+    make_tasks_share(tsk1, tsk2);
+    assert_group_cookie_not_equals_task_cookie(p, tsk1->pid_str);
+    assert_tasks_share(tsk1, tsk2);
+    assert_tasks_dont_share(tsk1, tsk3);
+    reset_task_cookie(tsk1);
+    reset_task_cookie(tsk2);
+
+    kill_task(tsk1);
+    kill_task(tsk2);
+    kill_task(tsk3);
+    del_group(p);
+    print_pass();
+}
+
+int main() {
+    char *root = make_group(NULL, NULL);
+
+    test_cgroup_parent_tag_child_inherit(root);
+    test_cgroup_parent_child_tag_inherit(root);
+    test_cgroup_coloring(root);
+    test_prctl_in_group(root);
+
+    del_root_group(root);
+    return 0;
+}
+
-- 
2.29.0.rc1.297.gfa9743e501-goog