linux-kernel - Re: [PATCH-mm v2] mm/list_lru: Optimize memcg_reparent_list_lru

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Date:   Tue, 8 Mar 2022 22:12:48 -0500
From:   Waiman Long <longman@...hat.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH-mm v2] mm/list_lru: Optimize
 memcg_reparent_list_lru_node()

On 3/8/22 21:13, Roman Gushchin wrote:
> On Tue, Mar 08, 2022 at 08:18:24PM -0500, Waiman Long wrote:
>> Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node()
>> to be race free"), we are tracking the total number of lru
>> entries in a list_lru_node in its nr_items field.  In the case of
>> memcg_reparent_list_lru_node(), there is nothing to be done if nr_items
>> is 0.  We don't even need to take the nlru->lock as no new lru entry
>> could be added by a racing list_lru_add() to the draining src_idx memcg
>> at this point.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   mm/list_lru.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/mm/list_lru.c b/mm/list_lru.c
>> index ba76428ceece..c669d87001a6 100644
>> --- a/mm/list_lru.c
>> +++ b/mm/list_lru.c
>> @@ -394,6 +394,12 @@ static void memcg_reparent_list_lru_node(struct list_lru *lru, int nid,
>>   	int dst_idx = dst_memcg->kmemcg_id;
>>   	struct list_lru_one *src, *dst;
>>   
>> +	/*
>> +	 * If there is no lru entry in this nlru, we can skip it immediately.
>> +	 */
>> +	if (!READ_ONCE(nlru->nr_items))
>> +		return;
> This is a per-node counter, not a per-memcg, right?
Right. list_lru_node is a per-node structure inside list_lru.
> If so, do we optimize for the case when all lru items belong to one node and
> others are empty?

That is actually the case that I am trying to optimize for.

If a system has many containers. It is also likely each container may 
mount one or more container specific filesystems. Since a container 
likely use just a few cpus, it is highly that only the list_lru_node 
that contains those cpus will be utilized while the rests may be empty.

I got the idea of doing this patch when I was looking at a crash dump 
related to the list_lru code. That particular crash dump has more than 
13k list_lru's and thousands of mount points. I had notice even if 
nr_items of a list_lru_node is 0, it still tries to transfer lru entries 
from source idx to dest idx. Without doing an lock/unlock and loading a 
cacheline from the memcg_lrus, it can save some time. That can be 
substantial saving if we are talking about thousands of list_lru's.

Cheers,
Longman