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selftests: cgroup: Replace sleep with cg_read_key_long_poll() for waiting on nr_dying_descendants 

Replace the manual sleep-and-retry logic in test_kmem_dead_cgroups()
with the new helper `cg_read_key_long_poll()`. This change improves
the robustness of the test by polling the "nr_dying_descendants"
counter in `cgroup.stat` until it reaches 0 or the timeout is exceeded.

Additionally, increase the retry timeout to 8 seconds (from 5 seconds)
based on testing results:
  - With 5-second timeout: 4/20 runs passed.
  - With 8-second timeout: 20/20 runs passed.

The 8 second timeout is based on stress testing of test_kmem_dead_cgroups()
under load: 5 seconds was occasionally not enough for reclaim of dying
descendants to complete, whereas 8 seconds consistently covered the observed
latencies. This value is intended as a generous upper bound for the
asynchronous reclaim and is not tied to any specific kernel constant, so it
can be adjusted in the future if reclaim behavior changes.

Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Guopeng Zhang 2025-12-03 19:56:31 +08:00 committed by Tejun Heo
parent 6360d444ae
commit 50133c09d1
1 changed files with 15 additions and 18 deletions
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33
tools/testing/selftests/cgroup/test_kmem.c
View File

@ -26,6 +26,7 @@
*/
#define MAX_VMSTAT_ERROR (4096 * 64 * get_nprocs())
#define KMEM_DEAD_WAIT_RETRIES 80
static int alloc_dcache(const char *cgroup, void *arg)
{
@ -306,9 +307,7 @@ static int test_kmem_dead_cgroups(const char *root)
{
int ret = KSFT_FAIL;
char *parent;
long dead;
int i;
int max_time = 20;
long dead = -1;
parent = cg_name(root, "kmem_dead_cgroups_test");
if (!parent)
@ -323,21 +322,19 @@ static int test_kmem_dead_cgroups(const char *root)
if (cg_run_in_subcgroups(parent, alloc_dcache, (void *)100, 30))
goto cleanup;
for (i = 0; i < max_time; i++) {
dead = cg_read_key_long(parent, "cgroup.stat",
"nr_dying_descendants ");
if (dead == 0) {
ret = KSFT_PASS;
break;
}
/*
* Reclaiming cgroups might take some time,
* let's wait a bit and repeat.
*/
sleep(1);
if (i > 5)
printf("Waiting time longer than 5s; wait: %ds (dead: %ld)\n", i, dead);
}
/*
* Allow up to ~8s for reclaim of dying descendants to complete.
* This is a generous upper bound derived from stress testing, not
* from a specific kernel constant, and can be adjusted if reclaim
* behavior changes in the future.
*/
dead = cg_read_key_long_poll(parent, "cgroup.stat",
"nr_dying_descendants ", 0, KMEM_DEAD_WAIT_RETRIES,
DEFAULT_WAIT_INTERVAL_US);
if (dead)
goto cleanup;
ret = KSFT_PASS;
cleanup:
cg_destroy(parent);