Re: [PATCH-mm v2] mm/list_lru: Optimize memcg_reparent_list_lru_node()

[Waiman Long <longman@redhat.com>]

On 3/8/22 23:46, Roman Gushchin wrote:
> On Tue, Mar 08, 2022 at 10:12:48PM -0500, Waiman Long wrote:
>> 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@redhat.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.
> Cool! Makes total sense to me. Thanks for the explanation!
> Would you mind to add this text to the commit log?
>
> Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Sure. I will add some of it into the commit log.

Cheers,
Longman