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Date:   Tue, 15 May 2018 06:54:15 +0300
From:   Vladimir Davydov <vdavydov.dev@gmail.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     akpm@linux-foundation.org, shakeelb@google.com,
        viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org,
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        aryabinin@virtuozzo.com
Subject: Re: [PATCH v5 03/13] mm: Assign memcg-aware shrinkers bitmap to memcg
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On Mon, May 14, 2018 at 12:34:45PM +0300, Kirill Tkhai wrote:
> >> +static void memcg_free_shrinker_maps(struct mem_cgroup *memcg)
> >> +{
> >> +	struct mem_cgroup_per_node *pn;
> >> +	struct memcg_shrinker_map *map;
> >> +	int nid;
> >> +
> >> +	if (memcg == root_mem_cgroup)
> >> +		return;
> >> +
> >> +	mutex_lock(&shrinkers_nr_max_mutex);
> > 
> > Why do you need to take the mutex here? You don't access shrinker map
> > capacity here AFAICS.
> 
> Allocation of shrinkers map is in css_online() now, and this wants its pay.
> memcg_expand_one_shrinker_map() must be able to differ mem cgroups with
> allocated maps, mem cgroups with not allocated maps, and mem cgroups with
> failed/failing css_online. So, the mutex is used for synchronization with
> expanding. See "old_size && !old" check in memcg_expand_one_shrinker_map().

Another reason to have 'expand' and 'alloc' paths separated - you
wouldn't need to take the mutex here as 'free' wouldn't be used for
undoing initial allocation, instead 'alloc' would cleanup by itself
while still holding the mutex.

> 
> >> +	for_each_node(nid) {
> >> +		pn = mem_cgroup_nodeinfo(memcg, nid);
> >> +		map = rcu_dereference_protected(pn->shrinker_map, true);
> >> +		if (map)
> >> +			call_rcu(&map->rcu, memcg_free_shrinker_map_rcu);
> >> +		rcu_assign_pointer(pn->shrinker_map, NULL);
> >> +	}
> >> +	mutex_unlock(&shrinkers_nr_max_mutex);
> >> +}
> >> +
> >> +static int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
> >> +{
> >> +	int ret, size = memcg_shrinker_nr_max/BITS_PER_BYTE;
> >> +
> >> +	if (memcg == root_mem_cgroup)
> >> +		return 0;
> >> +
> >> +	mutex_lock(&shrinkers_nr_max_mutex);
> >> +	ret = memcg_expand_one_shrinker_map(memcg, size, 0);
> > 
> > I don't think it's worth reusing the function designed for reallocating
> > shrinker maps for initial allocation. Please just fold the code here -
> > it will make both 'alloc' and 'expand' easier to follow IMHO.
> 
> These function will have 80% code the same. What are the reasons to duplicate
> the same functionality? Two functions are more difficult for support, and
> everywhere in kernel we try to avoid this IMHO.

IMHO two functions with clear semantics are easier to maintain than
a function that does one of two things depending on some condition.
Separating 'alloc' from 'expand' would only add 10-15 SLOC.

> >> +	mutex_unlock(&shrinkers_nr_max_mutex);
> >> +
> >> +	if (ret)
> >> +		memcg_free_shrinker_maps(memcg);
> >> +
> >> +	return ret;
> >> +}
> >> +
> >> +static struct idr mem_cgroup_idr;
> >> +
> >> +int memcg_expand_shrinker_maps(int old_nr, int nr)
> >> +{
> >> +	int size, old_size, ret = 0;
> >> +	struct mem_cgroup *memcg;
> >> +
> >> +	old_size = old_nr / BITS_PER_BYTE;
> >> +	size = nr / BITS_PER_BYTE;
> >> +
> >> +	mutex_lock(&shrinkers_nr_max_mutex);
> >> +
> >> +	if (!root_mem_cgroup)
> >> +		goto unlock;
> > 
> > This wants a comment.
> 
> Which comment does this want? "root_mem_cgroup is not initialized, so
> it does not have child mem cgroups"?

Looking at this code again, I find it pretty self-explaining, sorry.

Thanks.