Re: [PATCH] mm/memcg: bail out early when !memcg in mem_cgroup_lruvec

[Alex Shi <alex.shi@linux.alibaba.com>]

在 2020/11/28 下午12:02, Andrew Morton 写道:
> On Fri, 27 Nov 2020 11:08:35 +0800 Alex Shi <alex.shi@linux.alibaba.com> wrote:
> 
>> Sometime, we use NULL memcg in mem_cgroup_lruvec(memcg, pgdat)
>> so we could get out early in the situation to avoid useless checking.
>>
>> Also warning if both parameter are NULL.
> 
> Why do you think a warning is needed here?

Uh, Consider there are no problem for long time, it could be saved.

> 
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -613,14 +613,13 @@ static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
>>  	struct mem_cgroup_per_node *mz;
>>  	struct lruvec *lruvec;
>>  
>> -	if (mem_cgroup_disabled()) {
>> +	VM_WARN_ON_ONCE(!memcg && !pgdat);
>> +
>> +	if (mem_cgroup_disabled() || !memcg) {
>>  		lruvec = &pgdat->__lruvec;
>>  		goto out;
>>  	}
>>  
>> -	if (!memcg)
>> -		memcg = root_mem_cgroup;
>> -
> 
> This change isn't obviously equivalent, is it?

If !memcg, the root_mem_cgroup will still lead the lruvec to a pgdat
same as parameter.

> 
>>  	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
>>  	lruvec = &mz->lruvec;
>>  out:
> 
> And the resulting code is awkward:
> 
> 	if (mem_cgroup_disabled() || !memcg) {
> 		lruvec = &pgdat->__lruvec;
> 		goto out;
> 	}
> 
> 	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> 	lruvec = &mz->lruvec;
> out:
> 
> 
> could be
> 
> 	if (mem_cgroup_disabled() || !memcg) {
> 		lruvec = &pgdat->__lruvec;
> 	} else {
> 		mem_cgroup_per_node mz;
> 
> 		mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
> 		lruvec = &mz->lruvec;
> 	}
> 

Right. remove 'goto' is better for understander.

So, is the following patch ok?

From 225f29e03b40a7cbaeb4e3bb76f8efbcd7d648a2 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@linux.alibaba.com>
Date: Wed, 25 Nov 2020 14:06:33 +0800
Subject: [PATCH v2] mm/memcg: bail out early when !memcg in mem_cgroup_lruvec

Sometime, we use NULL memcg in mem_cgroup_lruvec(memcg, pgdat)
so we could get out early in the situation to avoid useless checking.

Polished as Andrew Morton's suggestion.

Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Cc: Yafang Shao <laoar.shao@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: linux-kernel@vger.kernel.org
---
 include/linux/memcontrol.h | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 3e6a1df3bdb9..4ff2ffe2b73d 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -610,20 +610,17 @@ mem_cgroup_nodeinfo(struct mem_cgroup *memcg, int nid)
 static inline struct lruvec *mem_cgroup_lruvec(struct mem_cgroup *memcg,
 					       struct pglist_data *pgdat)
 {
-	struct mem_cgroup_per_node *mz;
 	struct lruvec *lruvec;
 
-	if (mem_cgroup_disabled()) {
+	if (mem_cgroup_disabled() || !memcg) {
 		lruvec = &pgdat->__lruvec;
-		goto out;
-	}
+	} else {
+		struct mem_cgroup_per_node *mz;
 
-	if (!memcg)
-		memcg = root_mem_cgroup;
+		mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
+		lruvec = &mz->lruvec;
+	}
 
-	mz = mem_cgroup_nodeinfo(memcg, pgdat->node_id);
-	lruvec = &mz->lruvec;
-out:
 	/*
 	 * Since a node can be onlined after the mem_cgroup was created,
 	 * we have to be prepared to initialize lruvec->pgdat here;
-- 
2.29.GIT