Re: [PATCH v5 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Michal Koutný]

Hello.

On Mon, Apr 07, 2025 at 12:23:15PM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
> --- a/mm/memcontrol-v1.h
> +++ b/mm/memcontrol-v1.h
> @@ -22,8 +22,6 @@
>  	     iter != NULL;				\
>  	     iter = mem_cgroup_iter(NULL, iter, NULL))
>  
> -unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap);
> -

Hm, maybe keep it for v1 only where mem_cgroup_usage has meaning for
memsw (i.e. do the opposite and move the function definition to -v1.c).

>  void drain_all_stock(struct mem_cgroup *root_memcg);
>  
>  unsigned long memcg_events(struct mem_cgroup *memcg, int event);
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index b620d74b0f66..a771a0145a12 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -5963,6 +5963,10 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>  
>  		mem_cgroup_calculate_protection(target_memcg, memcg);
>  
> +		/* Skip memcg with no usage */
> +		if (!mem_cgroup_usage(memcg, false))
> +			continue;
> +

(Not only for v2), there is mem_cgroup_size() for this purpose (already
used in mm/vmscan.c).

>  		if (mem_cgroup_below_min(target_memcg, memcg)) {
>  			/*
>  			 * Hard protection.
> diff --git a/tools/testing/selftests/cgroup/test_memcontrol.c b/tools/testing/selftests/cgroup/test_memcontrol.c
> index 16f5d74ae762..bab826b6b7b0 100644
> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
> @@ -525,8 +525,13 @@ static int test_memcg_protection(const char *root, bool min)
>  		goto cleanup;
>  	}
>  
> +	/*
> +	 * Child 2 has memory.low=0, but some low protection is still being
> +	 * distributed down from its parent with memory.low=50M. So the low
> +	 * event count will be non-zero.
> +	 */
>  	for (i = 0; i < ARRAY_SIZE(children); i++) {
> -		int no_low_events_index = 1;
> +		int no_low_events_index = 2;

See suggestion in
https://lore.kernel.org/lkml/awgbdn6gwnj4kfaezsorvopgsdyoty3yahdeanqvoxstz2w2ke@xc3sv43elkz5/

HTH,
Michal

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