Re: [PATCH v6 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Michal Koutný]

On Sun, Apr 13, 2025 at 10:12:48PM -0400, Waiman Long <longman@xxxxxxxxxx> wrote:
> 2) memory.low is set to a non-zero value but the cgroup has no task in
>    it so that it has an effective low value of 0. Again it may have a
>    non-zero low event count if memory reclaim happens. This is probably
>    not a result expected by the users and it is really doubtful that
>    users will check an empty cgroup with no task in it and expecting
>    some non-zero event counts.

I think you want to distinguish "no tasks" vs "no usage" in this
paragraph.


> --- 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;
> +
>  		if (mem_cgroup_below_min(target_memcg, memcg)) {

As I think more about this -- the idea expressed by the diff makes
sense. But is it really a change?
For non-root memcgs, they'll be skipped because 0 >= 0 (in
mem_cgroup_below_min()) and root memcg would hardly be skipped.


> --- a/tools/testing/selftests/cgroup/test_memcontrol.c
> +++ b/tools/testing/selftests/cgroup/test_memcontrol.c
> @@ -380,10 +380,10 @@ static bool reclaim_until(const char *memcg, long goal);
>   *
>   * Then it checks actual memory usages and expects that:
>   * A/B    memory.current ~= 50M
> - * A/B/C  memory.current ~= 29M
> - * A/B/D  memory.current ~= 21M
> - * A/B/E  memory.current ~= 0
> - * A/B/F  memory.current  = 0
> + * A/B/C  memory.current ~= 29M [memory.events:low > 0]
> + * A/B/D  memory.current ~= 21M [memory.events:low > 0]
> + * A/B/E  memory.current ~= 0   [memory.events:low == 0 if !memory_recursiveprot, > 0 otherwise]

Please note the subtlety in my suggestion -- I want the test with
memory_recursiveprot _not_ to check events count at all. Because:
	a) it forces single interpretation of low events wrt effective
	   low limit 
	b) effective low limit should still be 0 in E in this testcase
	   (there should be no unclaimed protection of C and D).

> + * A/B/F  memory.current  = 0   [memory.events:low == 0]


Thanks,
Michal

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