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

[Waiman Long <llong@redhat.com>]

On 4/14/25 8:42 AM, Michal Koutný wrote:
> On Sun, Apr 13, 2025 at 10:12:48PM -0400, Waiman Long <longman@redhat.com> 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.

Good point. Will update it if I need to send a new version.


>> --- 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.

I did see some low event in the no usage case because of the ">=" 
comparison used in mem_cgroup_below_min(). I originally planning to 
guard against the elow == 0 case but Johannes advised against it.


>
>
>> --- 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).

Yes, low event count for E is 0 in the !memory_recursiveprot case, but 
C/D still have low events and setting no_low_events_index to -1 will 
fail the test and it is not the same as not checking low event counts at 
all.

Cheers,
Longman