Test cgroup v2 freezer time stat. Freezer time accounting should
be independent of other cgroups in the hierarchy and should increase
iff a cgroup is CGRP_FREEZE (regardless of whether it reaches
CGRP_FROZEN).

Skip these tests on systems without freeze time accounting.

Signed-off-by: Tiffany Yang <ynaffit@google.com>
---
 tools/testing/selftests/cgroup/test_freezer.c | 686 ++++++++++++++++++
 1 file changed, 686 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_freezer.c b/tools/testing/selftests/cgroup/test_freezer.c
index 8730645d363a..c0880ecfa814 100644
--- a/tools/testing/selftests/cgroup/test_freezer.c
+++ b/tools/testing/selftests/cgroup/test_freezer.c
@@ -804,6 +804,685 @@ static int test_cgfreezer_vfork(const char *root)
 	return ret;
 }
 
+/*
+ * Get the current freeze_time_total for the cgroup.
+ */
+static long cg_check_freezetime(const char *cgroup)
+{
+	return cg_read_key_long(cgroup, "cgroup.freeze.stat.local",
+				"freeze_time_total ");
+}
+
+/*
+ * Test that the freeze time will behave as expected for an empty cgroup.
+ */
+static int test_cgfreezer_time_empty(const char *root)
+{
+	int ret = KSFT_FAIL;
+	char *cgroup = NULL;
+	long prev, curr;
+	int i;
+
+	cgroup = cg_name(root, "cg_time_test_empty");
+	if (!cgroup)
+		goto cleanup;
+
+	/*
+	 * 1) Create an empty cgroup and check that its freeze time
+	 *    is 0.
+	 */
+	if (cg_create(cgroup))
+		goto cleanup;
+
+	curr = cg_check_freezetime(cgroup);
+	if (curr) {
+		if (curr < 0)
+			ret = KSFT_SKIP;
+		else
+			debug("Expect time (%ld) to be 0\n", curr);
+
+		goto cleanup;
+	}
+
+	/*
+	 * 2) Freeze the cgroup. Check that its freeze time is
+	 *    larger than 0.
+	 */
+	if (cg_freeze_nowait(cgroup, true))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) > 0\n", curr);
+		goto cleanup;
+	}
+
+	/*
+	 * 3) Sleep for 100 us. Check that the freeze time is at
+	 *    least 100 us larger than it was at 2).
+	 */
+	usleep(100);
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if ((curr - prev) < 100) {
+		debug("Expect time (%ld) to be at least 100 us more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 4) Unfreeze the cgroup. Check that the freeze time is
+	 *    larger than at 3).
+	 */
+	if (cg_freeze_nowait(cgroup, false))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 5) Check the freeze time again to ensure that it has not
+	 *    changed.
+	 */
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr != prev) {
+		debug("Expect time (%ld) to be unchanged from previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	if (cgroup)
+		cg_destroy(cgroup);
+	free(cgroup);
+	return ret;
+}
+
+/*
+ * A simple test for cgroup freezer time accounting. This test follows
+ * the same flow as test_cgfreezer_time_empty, but with a single process
+ * in the cgroup.
+ */
+static int test_cgfreezer_time_simple(const char *root)
+{
+	int ret = KSFT_FAIL;
+	char *cgroup = NULL;
+	long prev, curr;
+	int i;
+
+	cgroup = cg_name(root, "cg_time_test_simple");
+	if (!cgroup)
+		goto cleanup;
+
+	/*
+	 * 1) Create a cgroup and check that its freeze time is 0.
+	 */
+	if (cg_create(cgroup))
+		goto cleanup;
+
+	curr = cg_check_freezetime(cgroup);
+	if (curr) {
+		if (curr < 0)
+			ret = KSFT_SKIP;
+		else
+			debug("Expect time (%ld) to be 0\n", curr);
+
+		goto cleanup;
+	}
+
+	/*
+	 * 2) Populate the cgroup with one child and check that the
+	 *    freeze time is still 0.
+	 */
+	cg_run_nowait(cgroup, child_fn, NULL);
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr > prev) {
+		debug("Expect time (%ld) to be 0\n", curr);
+		goto cleanup;
+	}
+
+	/*
+	 * 3) Freeze the cgroup. Check that its freeze time is
+	 *    larger than 0.
+	 */
+	if (cg_freeze_nowait(cgroup, true))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) > 0\n", curr);
+		goto cleanup;
+	}
+
+	/*
+	 * 4) Sleep for 100 us. Check that the freeze time is at
+	 *    least 100 us larger than it was at 3).
+	 */
+	usleep(100);
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if ((curr - prev) < 100) {
+		debug("Expect time (%ld) to be at least 100 us more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 5) Unfreeze the cgroup. Check that the freeze time is
+	 *    larger than at 4).
+	 */
+	if (cg_freeze_nowait(cgroup, false))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 6) Sleep for 100 us. Check that the freeze time is the
+	 *    same as at 5).
+	 */
+	usleep(100);
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr != prev) {
+		debug("Expect time (%ld) to be unchanged from previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	if (cgroup)
+		cg_destroy(cgroup);
+	free(cgroup);
+	return ret;
+}
+
+/*
+ * Test that freezer time accounting works as expected, even while we're
+ * populating a cgroup with processes.
+ */
+static int test_cgfreezer_time_populate(const char *root)
+{
+	int ret = KSFT_FAIL;
+	char *cgroup = NULL;
+	long prev, curr;
+	int i;
+
+	cgroup = cg_name(root, "cg_time_test_populate");
+	if (!cgroup)
+		goto cleanup;
+
+	if (cg_create(cgroup))
+		goto cleanup;
+
+	curr = cg_check_freezetime(cgroup);
+	if (curr) {
+		if (curr < 0)
+			ret = KSFT_SKIP;
+		else
+			debug("Expect time (%ld) to be 0\n", curr);
+
+		goto cleanup;
+	}
+
+	/*
+	 * 1) Populate the cgroup with 100 processes. Check that
+	 *    the freeze time is 0.
+	 */
+	for (i = 0; i < 100; i++)
+		cg_run_nowait(cgroup, child_fn, NULL);
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr != prev) {
+		debug("Expect time (%ld) to be 0\n", curr);
+		goto cleanup;
+	}
+
+	/*
+	 * 2) Wait for the group to become fully populated. Check
+	 *    that the freeze time is 0.
+	 */
+	if (cg_wait_for_proc_count(cgroup, 100))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr != prev) {
+		debug("Expect time (%ld) to be 0\n", curr);
+		goto cleanup;
+	}
+
+	/*
+	 * 3) Freeze the cgroup and then populate it with 100 more
+	 *    processes. Check that the freeze time continues to grow.
+	 */
+	if (cg_freeze_nowait(cgroup, true))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	for (i = 0; i < 100; i++)
+		cg_run_nowait(cgroup, child_fn, NULL);
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 4) Wait for the group to become fully populated. Check
+	 *    that the freeze time is larger than at 3).
+	 */
+	if (cg_wait_for_proc_count(cgroup, 200))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 5) Unfreeze the cgroup. Check that the freeze time is
+	 *    larger than at 4).
+	 */
+	if (cg_freeze_nowait(cgroup, false))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 6) Kill the processes. Check that the freeze time is the
+	 *    same as it was at 5).
+	 */
+	if (cg_killall(cgroup))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr != prev) {
+		debug("Expect time (%ld) to be unchanged from previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	/*
+	 * 7) Freeze and unfreeze the cgroup. Check that the freeze
+	 *    time is larger than it was at 6).
+	 */
+	if (cg_freeze_nowait(cgroup, true))
+		goto cleanup;
+	if (cg_freeze_nowait(cgroup, false))
+		goto cleanup;
+	prev = curr;
+	curr = cg_check_freezetime(cgroup);
+	if (curr <= prev) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr, prev);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	if (cgroup)
+		cg_destroy(cgroup);
+	free(cgroup);
+	return ret;
+}
+
+/*
+ * Test that frozen time for a cgroup continues to work as expected,
+ * even as processes are migrated. Frozen cgroup A's freeze time should
+ * continue to increase and running cgroup B's should stay 0.
+ */
+static int test_cgfreezer_time_migrate(const char *root)
+{
+	long prev_A, curr_A, curr_B;
+	char *cgroup[2] = {0};
+	int ret = KSFT_FAIL;
+	int pid, i;
+
+	cgroup[0] = cg_name(root, "cg_time_test_migrate_A");
+	if (!cgroup[0])
+		goto cleanup;
+
+	cgroup[1] = cg_name(root, "cg_time_test_migrate_B");
+	if (!cgroup[1])
+		goto cleanup;
+
+	if (cg_create(cgroup[0]))
+		goto cleanup;
+
+	if (cg_check_freezetime(cgroup[0]) < 0) {
+		ret = KSFT_SKIP;
+		goto cleanup;
+	}
+
+	if (cg_create(cgroup[1]))
+		goto cleanup;
+
+	pid = cg_run_nowait(cgroup[0], child_fn, NULL);
+	if (pid < 0)
+		goto cleanup;
+
+	if (cg_wait_for_proc_count(cgroup[0], 1))
+		goto cleanup;
+
+	curr_A = cg_check_freezetime(cgroup[0]);
+	if (curr_A) {
+		debug("Expect time (%ld) to be 0\n", curr_A);
+		goto cleanup;
+	}
+	curr_B = cg_check_freezetime(cgroup[1]);
+	if (curr_B) {
+		debug("Expect time (%ld) to be 0\n", curr_B);
+		goto cleanup;
+	}
+
+	/*
+	 * Freeze cgroup A.
+	 */
+	if (cg_freeze_wait(cgroup[0], true))
+		goto cleanup;
+	prev_A = curr_A;
+	curr_A = cg_check_freezetime(cgroup[0]);
+	if (curr_A <= prev_A) {
+		debug("Expect time (%ld) to be > 0\n", curr_A);
+		goto cleanup;
+	}
+
+	/*
+	 * Migrate from A (frozen) to B (running).
+	 */
+	if (cg_enter(cgroup[1], pid))
+		goto cleanup;
+
+	usleep(1000);
+	curr_B = cg_check_freezetime(cgroup[1]);
+	if (curr_B) {
+		debug("Expect time (%ld) to be 0\n", curr_B);
+		goto cleanup;
+	}
+
+	prev_A = curr_A;
+	curr_A = cg_check_freezetime(cgroup[0]);
+	if (curr_A <= prev_A) {
+		debug("Expect time (%ld) to be more than previous check (%ld)\n",
+		      curr_A, prev_A);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	if (cgroup[0])
+		cg_destroy(cgroup[0]);
+	free(cgroup[0]);
+	if (cgroup[1])
+		cg_destroy(cgroup[1]);
+	free(cgroup[1]);
+	return ret;
+}
+
+/*
+ * The test creates a cgroup and freezes it. Then it creates a child cgroup.
+ * After that it checks that the child cgroup has a non-zero freeze time
+ * that is less than the parent's. Next, it freezes the child, unfreezes
+ * the parent, and sleeps. Finally, it checks that the child's freeze
+ * time has grown larger than the parent's.
+ */
+static int test_cgfreezer_time_parent(const char *root)
+{
+	char *parent, *child = NULL;
+	int ret = KSFT_FAIL;
+	long ptime, ctime;
+
+	parent = cg_name(root, "cg_test_parent_A");
+	if (!parent)
+		goto cleanup;
+
+	child = cg_name(parent, "cg_test_parent_B");
+	if (!child)
+		goto cleanup;
+
+	if (cg_create(parent))
+		goto cleanup;
+
+	if (cg_check_freezetime(parent) < 0) {
+		ret = KSFT_SKIP;
+		goto cleanup;
+	}
+
+	if (cg_freeze_wait(parent, true))
+		goto cleanup;
+
+	usleep(1000);
+	if (cg_create(child))
+		goto cleanup;
+
+	if (cg_check_frozen(child, true))
+		goto cleanup;
+
+	/*
+	 * Since the parent was frozen the entire time the child cgroup
+	 * was being created, we expect the parent's freeze time to be
+	 * larger than the child's.
+	 *
+	 * Ideally, we would be able to check both times simultaneously,
+	 * but here we get the child's after we get the parent's.
+	 */
+	ptime = cg_check_freezetime(parent);
+	ctime = cg_check_freezetime(child);
+	if (ptime <= ctime) {
+		debug("Expect ptime (%ld) > ctime (%ld)\n", ptime, ctime);
+		goto cleanup;
+	}
+
+	if (cg_freeze_nowait(child, true))
+		goto cleanup;
+
+	if (cg_freeze_wait(parent, false))
+		goto cleanup;
+
+	if (cg_check_frozen(child, true))
+		goto cleanup;
+
+	usleep(100000);
+
+	ctime = cg_check_freezetime(child);
+	ptime = cg_check_freezetime(parent);
+
+	if (ctime <= ptime) {
+		debug("Expect ctime (%ld) > ptime (%ld)\n", ctime, ptime);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	if (child)
+		cg_destroy(child);
+	free(child);
+	if (parent)
+		cg_destroy(parent);
+	free(parent);
+	return ret;
+}
+
+/*
+ * The test creates a parent cgroup and a child cgroup. Then, it freezes
+ * the child and checks that the child's freeze time is greater than the
+ * parent's, which should be zero.
+ */
+static int test_cgfreezer_time_child(const char *root)
+{
+	char *parent, *child = NULL;
+	int ret = KSFT_FAIL;
+	long ptime, ctime;
+
+	parent = cg_name(root, "cg_test_child_A");
+	if (!parent)
+		goto cleanup;
+
+	child = cg_name(parent, "cg_test_child_B");
+	if (!child)
+		goto cleanup;
+
+	if (cg_create(parent))
+		goto cleanup;
+
+	if (cg_check_freezetime(parent) < 0) {
+		ret = KSFT_SKIP;
+		goto cleanup;
+	}
+
+	if (cg_create(child))
+		goto cleanup;
+
+	if (cg_freeze_wait(child, true))
+		goto cleanup;
+
+	ctime = cg_check_freezetime(child);
+	ptime = cg_check_freezetime(parent);
+	if (ptime != 0) {
+		debug("Expect ptime (%ld) to be 0\n", ptime);
+		goto cleanup;
+	}
+
+	if (ctime <= ptime) {
+		debug("Expect ctime (%ld) <= ptime (%ld)\n", ctime, ptime);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	if (child)
+		cg_destroy(child);
+	free(child);
+	if (parent)
+		cg_destroy(parent);
+	free(parent);
+	return ret;
+}
+
+/*
+ * The test creates the following hierarchy:
+ *    A
+ *    |
+ *    B
+ *    |
+ *    C
+ *
+ * Then it freezes the cgroups in the order C, B, A.
+ * Then it unfreezes the cgroups in the order A, B, C.
+ * Then it checks that C's freeze time is larger than B's and
+ * that B's is larger than A's.
+ */
+static int test_cgfreezer_time_nested(const char *root)
+{
+	char *cgroup[3] = {0};
+	int ret = KSFT_FAIL;
+	long time[3] = {0};
+	int i;
+
+	cgroup[0] = cg_name(root, "cg_test_time_A");
+	if (!cgroup[0])
+		goto cleanup;
+
+	cgroup[1] = cg_name(cgroup[0], "B");
+	if (!cgroup[1])
+		goto cleanup;
+
+	cgroup[2] = cg_name(cgroup[1], "C");
+	if (!cgroup[2])
+		goto cleanup;
+
+	if (cg_create(cgroup[0]))
+		goto cleanup;
+
+	if (cg_check_freezetime(cgroup[0]) < 0) {
+		ret = KSFT_SKIP;
+		goto cleanup;
+	}
+
+	if (cg_create(cgroup[1]))
+		goto cleanup;
+
+	if (cg_create(cgroup[2]))
+		goto cleanup;
+
+	if (cg_freeze_nowait(cgroup[2], true))
+		goto cleanup;
+
+	if (cg_freeze_nowait(cgroup[1], true))
+		goto cleanup;
+
+	if (cg_freeze_nowait(cgroup[0], true))
+		goto cleanup;
+
+	usleep(1000);
+
+	if (cg_freeze_nowait(cgroup[0], false))
+		goto cleanup;
+
+	if (cg_freeze_nowait(cgroup[1], false))
+		goto cleanup;
+
+	if (cg_freeze_nowait(cgroup[2], false))
+		goto cleanup;
+
+	time[2] = cg_check_freezetime(cgroup[2]);
+	time[1] = cg_check_freezetime(cgroup[1]);
+	time[0] = cg_check_freezetime(cgroup[0]);
+
+	if (time[2] <= time[1]) {
+		debug("Expect C's time (%ld) > B's time (%ld)", time[2], time[1]);
+		goto cleanup;
+	}
+
+	if (time[1] <= time[0]) {
+		debug("Expect B's time (%ld) > A's time (%ld)", time[1], time[0]);
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+
+cleanup:
+	for (i = 2; i >= 0 && cgroup[i]; i--) {
+		cg_destroy(cgroup[i]);
+		free(cgroup[i]);
+	}
+
+	return ret;
+}
+
 #define T(x) { x, #x }
 struct cgfreezer_test {
 	int (*fn)(const char *root);
@@ -819,6 +1498,13 @@ struct cgfreezer_test {
 	T(test_cgfreezer_stopped),
 	T(test_cgfreezer_ptraced),
 	T(test_cgfreezer_vfork),
+	T(test_cgfreezer_time_empty),
+	T(test_cgfreezer_time_simple),
+	T(test_cgfreezer_time_populate),
+	T(test_cgfreezer_time_migrate),
+	T(test_cgfreezer_time_parent),
+	T(test_cgfreezer_time_child),
+	T(test_cgfreezer_time_nested),
 };
 #undef T
 
-- 
2.50.1.565.gc32cd1483b-goog

On Mon, Aug 04, 2025 at 08:29:42PM -0700, Tiffany Yang <ynaffit@google.com> wrote:

> +static int test_cgfreezer_time_empty(const char *root)
> +{
> +	int ret = KSFT_FAIL;
> +	char *cgroup = NULL;
> +	long prev, curr;
> +	int i;
> +
> +	cgroup = cg_name(root, "cg_time_test_empty");
> +	if (!cgroup)
> +		goto cleanup;
> +
> +	/*
> +	 * 1) Create an empty cgroup and check that its freeze time
> +	 *    is 0.
> +	 */
> +	if (cg_create(cgroup))
> +		goto cleanup;
> +
> +	curr = cg_check_freezetime(cgroup);
> +	if (curr) {
> +		if (curr < 0)
> +			ret = KSFT_SKIP;
> +		else
> +			debug("Expect time (%ld) to be 0\n", curr);
> +
> +		goto cleanup;
> +	}

	if (curr < 0) {
		ret = KSFT_SKIP;
		goto cleanup;
	}
	if (curr > 0) {
		debug("Expect time (%ld) to be 0\n", curr);
		goto cleanup;
	}

I might like the version with less indentation and explicit guards. It's
only minor stylistic issue.

> +
> +	/*
> +	 * 2) Freeze the cgroup. Check that its freeze time is
> +	 *    larger than 0.
> +	 */
> +	if (cg_freeze_nowait(cgroup, true))
> +		goto cleanup;
> +	prev = curr;
> +	curr = cg_check_freezetime(cgroup);
> +	if (curr <= prev) {

Here and...
> +		debug("Expect time (%ld) > 0\n", curr);
> +		goto cleanup;
> +	}
> +
> +	/*
> +	 * 3) Sleep for 100 us. Check that the freeze time is at
> +	 *    least 100 us larger than it was at 2).
> +	 */
> +	usleep(100);
> +	prev = curr;
> +	curr = cg_check_freezetime(cgroup);
> +	if ((curr - prev) < 100) {

...here
I'm slightly worried it may cause test flakiness on systems with too
coarse clock granularity.

Is the first check anyhow meaningful? (I think it's only as strong as
checking return value of the preceding write(2) to cgroup.freeze.)

Would it compromise your use case if the latter check was at least
1000 μs (based on other usleeps in cgroup selftests)? (Ditto for other
100 μs checks.)

Or does anything guarantee the minimal precision in common selftest
environments?


Thanks,
Michal

Michal Koutný <mkoutny@suse.com> writes:
...


> 	if (curr < 0) {
> 		ret = KSFT_SKIP;
> 		goto cleanup;
> 	}
> 	if (curr > 0) {
> 		debug("Expect time (%ld) to be 0\n", curr);
> 		goto cleanup;
> 	}

> I might like the version with less indentation and explicit guards. It's
> only minor stylistic issue.


Noted! Will be fixed in v4.

>> +
>> +	/*
>> +	 * 2) Freeze the cgroup. Check that its freeze time is
>> +	 *    larger than 0.
>> +	 */
>> +	if (cg_freeze_nowait(cgroup, true))
>> +		goto cleanup;
>> +	prev = curr;
>> +	curr = cg_check_freezetime(cgroup);
>> +	if (curr <= prev) {

> Here and...
>> +		debug("Expect time (%ld) > 0\n", curr);
>> +		goto cleanup;
>> +	}
>> +
>> +	/*
>> +	 * 3) Sleep for 100 us. Check that the freeze time is at
>> +	 *    least 100 us larger than it was at 2).
>> +	 */
>> +	usleep(100);
>> +	prev = curr;
>> +	curr = cg_check_freezetime(cgroup);
>> +	if ((curr - prev) < 100) {

> ...here
> I'm slightly worried it may cause test flakiness on systems with too
> coarse clock granularity.

> Is the first check anyhow meaningful? (I think it's only as strong as
> checking return value of the preceding write(2) to cgroup.freeze.)


Hmm I had originally put the check at 2) in to make sure that the value
increases as expected for an empty cgroup (the simplest case), but I
think the check at 3) (and most other checks in these test cases)
establish the same thing.

The other purpose it serves is to act as kind of a buffer for the time
it takes to freeze the cgroup (t_1 -> t_2) to ensure that the cgroup
would be frozen for the entirety of the sleep. I.e., preventing the case
where we fail the check because the time measured at t_3 ends up being
(100 - the time it took to freeze).

That said, the time between writing to an empty cgroup's cgroup.freeze
and it beginning to freeze is basically negligible relative to the time
scales in this test, so I'm happy to take it out!

(If what I've written above is worded too confusingly, ignore it.
TL;DR: we don't need this check! I'm taking it out!)

> Would it compromise your use case if the latter check was at least
> 1000 μs (based on other usleeps in cgroup selftests)? (Ditto for other
> 100 μs checks.)

Not at all! I'll make this change for v4.

Thanks,
-- 
Tiffany Y. Yang