Re: [PATCH] mm/vmscan: get number of pages on the LRU list in memcgroup base on lru_zone_size

[Honglei Wang <honglei.wang@oracle.com>]

On 9/3/19 5:28 PM, Michal Hocko wrote:
> On Tue 03-09-19 16:54:16, Honglei Wang wrote:
>> lruvec_lru_size() is involving lruvec_page_state_local() to get the
>> lru_size in the current code. It's base on lruvec_stat_local.count[]
>> of mem_cgroup_per_node. This counter is updated in batch. It won't
>> do charge if the number of coming pages doesn't meet the needs of
>> MEMCG_CHARGE_BATCH who's defined as 32 now.
>>
>> This causes small section of memory can't be handled as expected in
>> some scenario. For example, if we have only 32 pages madvise free
>> memory in memcgroup, these pages won't be freed as expected when it
>> meets memory pressure in this group.
> 
> Could you be more specific please?

Okay, will add more detailed description at next version.

> 
>> Getting lru_size base on lru_zone_size of mem_cgroup_per_node which
>> is not updated in batch can make this a bit more accurate.
> 
> This is effectivelly reverting 1a61ab8038e72. There were no numbers
> backing that commit, neither this one has. The only hot path I can see
> is workingset_refault. All others seems to be in the reclaim path.

Yep, I saw the lruvec increasing is not in batch way in 
workingset_refault path, so thought maybe change the way in 
lruvec_lru_size() to no-batch might match the act more.

It seems to me less than 32 pages deviations for seq_show stuff is not a 
big deal (maybe it's not a big deal in lruvec_lru_size() as well...), so 
I choose just modify lruvec_lru_size(), but not revert 1a61ab8038e72.

>   
>> Signed-off-by: Honglei Wang <honglei.wang@oracle.com>
> 
> That being said, I am not against this patch but the changelog should be
> more specific about the particular problem and how serious it is.

Thanks for the suggestions. I don't think this is a really serious one, 
and I'll try to give a more detailed changelog about what it's trying to 
fix.

Honglei

> 
>> ---
>>   mm/vmscan.c | 9 +++++----
>>   1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index c77d1e3761a7..c28672460868 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -354,12 +354,13 @@ unsigned long zone_reclaimable_pages(struct zone *zone)
>>    */
>>   unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone_idx)
>>   {
>> -	unsigned long lru_size;
>> +	unsigned long lru_size = 0;
>>   	int zid;
>>   
>> -	if (!mem_cgroup_disabled())
>> -		lru_size = lruvec_page_state_local(lruvec, NR_LRU_BASE + lru);
>> -	else
>> +	if (!mem_cgroup_disabled()) {
>> +		for (zid = 0; zid < MAX_NR_ZONES; zid++)
>> +			lru_size += mem_cgroup_get_zone_lru_size(lruvec, lru, zid);
>> +	} else
>>   		lru_size = node_page_state(lruvec_pgdat(lruvec), NR_LRU_BASE + lru);
>>   
>>   	for (zid = zone_idx + 1; zid < MAX_NR_ZONES; zid++) {
>> -- 
>> 2.17.0
>