Re: [PATCH 2/2] selftests/cgroup: include slab in test_percpu_basic memory check

[Waiman Long <longman@redhat.com>]

On 3/6/26 2:18 AM, Li Wang wrote:
> test_percpu_basic() currently compares memory.current against only
> memory.stat:percpu after creating 1000 child cgroups.
>
> Observed failure:
>    #./test_kmem
>    ok 1 test_kmem_basic
>    ok 2 test_kmem_memcg_deletion
>    ok 3 test_kmem_proc_kpagecgroup
>    ok 4 test_kmem_kernel_stacks
>    ok 5 test_kmem_dead_cgroups
>    memory.current 11530240
>    percpu 8440000
>    not ok 6 test_percpu_basic
>
> That assumption is too strict: child cgroup creation also allocates
> slab-backed metadata, so memory.current is expected to be larger than
> percpu alone. One visible path is:
>
>    cgroup_mkdir()
>      cgroup_create()
>        cgroup_addrm_file()
>          cgroup_add_file()
>            __kernfs_create_file()
>              __kernfs_new_node()
>                kmem_cache_zalloc()
>
> These kernfs allocations are charged as slab and show up in
> memory.stat:slab.
>
> Update the check to compare memory.current against (percpu + slab)
> within MAX_VMSTAT_ERROR, and print slab/delta in the failure message to
> improve diagnostics.
>
> Signed-off-by: Li Wang <liwang@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Shakeel Butt <shakeelb@google.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> ---
>   tools/testing/selftests/cgroup/test_kmem.c | 11 ++++++-----
>   1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/tools/testing/selftests/cgroup/test_kmem.c b/tools/testing/selftests/cgroup/test_kmem.c
> index 249d79113067..4b5799698893 100644
> --- a/tools/testing/selftests/cgroup/test_kmem.c
> +++ b/tools/testing/selftests/cgroup/test_kmem.c
> @@ -353,7 +353,7 @@ static int test_percpu_basic(const char *root)
>   {
>   	int ret = KSFT_FAIL;
>   	char *parent, *child;
> -	long current, percpu;
> +	long current, percpu, slab;
>   	int i;
>   
>   	parent = cg_name(root, "percpu_basic_test");
> @@ -379,13 +379,14 @@ static int test_percpu_basic(const char *root)
>   
>   	current = cg_read_long(parent, "memory.current");
>   	percpu = cg_read_key_long(parent, "memory.stat", "percpu ");
> +	slab = cg_read_key_long(parent, "memory.stat", "slab ");
>   
> -	if (current > 0 && percpu > 0 && labs(current - percpu) <
> -	    MAX_VMSTAT_ERROR)
> +	if (current > 0 && percpu > 0 && slab >= 0 &&
> +			labs(current - (percpu + slab)) < MAX_VMSTAT_ERROR)
>   		ret = KSFT_PASS;
>   	else
> -		printf("memory.current %ld\npercpu %ld\n",
> -		       current, percpu);
> +		printf("memory.current %ld\npercpu %ld\nslab %ld\ndelta %ld\n",
> +			current, percpu, slab, current - (percpu + slab));
>   
>   cleanup_children:
>   	for (i = 0; i < 1000; i++) {
Reviewed-by: Waiman Long <longman@redhat.com>