Re: [PATCH v4 1/2] mm/vmscan: Skip memcg with !usage in shrink_node_memcgs()

[Waiman Long <llong@redhat.com>]

On 4/7/25 10:24 AM, Johannes Weiner wrote:
> On Sun, Apr 06, 2025 at 09:41:58PM -0400, Waiman Long wrote:
>> The test_memcontrol selftest consistently fails its test_memcg_low
>> sub-test due to the fact that two of its test child cgroups which
>> have a memmory.low of 0 or an effective memory.low of 0 still have low
>> events generated for them since mem_cgroup_below_low() use the ">="
>> operator when comparing to elow.
>>
>> The two failed use cases are as follows:
>>
>> 1) memory.low is set to 0, but low events can still be triggered and
>>     so the cgroup may have a non-zero low event count. I doubt users are
>>     looking for that as they didn't set memory.low at all.
>>
>> 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.
>>
>> In the first case, even though memory.low isn't set, it may still have
>> some low protection if memory.low is set in the parent. So low event may
>> still be recorded. The test_memcontrol.c test has to be modified to
>> account for that.
>>
>> For the second case, it really doesn't make sense to have non-zero
>> low event if the cgroup has 0 usage. So we need to skip this corner
>> case in shrink_node_memcgs() by skipping the !usage case. The
>> "#ifdef CONFIG_MEMCG" directive is added to avoid problem with the
>> non-CONFIG_MEMCG case.
>>
>> With this patch applied, the test_memcg_low sub-test finishes
>> successfully without failure in most cases. Though both test_memcg_low
>> and test_memcg_min sub-tests may still fail occasionally if the
>> memory.current values fall outside of the expected ranges.
>>
>> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   mm/vmscan.c                                      | 10 ++++++++++
>>   tools/testing/selftests/cgroup/test_memcontrol.c |  7 ++++++-
>>   2 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index b620d74b0f66..65dee0ad6627 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -5926,6 +5926,7 @@ static inline bool should_continue_reclaim(struct pglist_data *pgdat,
>>   	return inactive_lru_pages > pages_for_compaction;
>>   }
>>   
>> +#ifdef CONFIG_MEMCG
>>   static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>>   {
>>   	struct mem_cgroup *target_memcg = sc->target_mem_cgroup;
>> @@ -5963,6 +5964,10 @@ 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 (!page_counter_read(&memcg->memory))
>> +			continue;
> Please use mem_cgroup_usage() like I had originally suggested.
>
> The !CONFIG_MEMCG case can be done like its root cgroup branch.
Will do that.
>
>>   		if (mem_cgroup_below_min(target_memcg, memcg)) {
>>   			/*
>>   			 * Hard protection.
>> @@ -6004,6 +6009,11 @@ static void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>>   		}
>>   	} while ((memcg = mem_cgroup_iter(target_memcg, memcg, partial)));
>>   }
>> +#else
>> +static inline void shrink_node_memcgs(pg_data_t *pgdat, struct scan_control *sc)
>> +{
>> +}
>> +#endif /* CONFIG_MEMCG */
> You made the entire reclaim path a nop for !CONFIG_MEMCG.

Yes, that is probably not right. Will fix that.

Cheers,
Longman