linux-kernel - Re: [PATCH v5 1/2] mm/vmscan: Skip memcg with !usage in shrink_node

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Message-ID: <0158e459-c205-4a88-9711-3dea2bca71ae@redhat.com>
Date: Fri, 11 Apr 2025 17:13:00 -0400
From: Waiman Long <llong@...hat.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, linux-mm@...ck.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v5 1/2] mm/vmscan: Skip memcg with !usage in
 shrink_node_memcgs()


On 4/11/25 1:11 PM, Michal Koutný wrote:
> Hello.
>
> On Mon, Apr 07, 2025 at 12:23:15PM -0400, Waiman Long <longman@...hat.com> 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).
memcontrol-v1.c also include mm/internal.h. That is the reason why I can 
remove it from here.
>>   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).
My understanding is that mem_cgroup_usage() is for both v1 and v2, while 
mem_cgroup_size() is for v2 only.
>
>>   		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/

I have just replied on your suggestion.

Cheers,
Longman

>
> HTH,
> Michal