linux-kernel - Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless

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Date:   Wed, 22 Feb 2023 22:58:37 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     sultan@...neltoast.com, dave@...olabs.net,
        penguin-kernel@...ove.SAKURA.ne.jp, paulmck@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH 2/5] mm: vmscan: make memcg slab shrink lockless

On 22.02.2023 10:32, Qi Zheng wrote:
> 
> 
> On 2023/2/22 05:28, Kirill Tkhai wrote:
>> On 20.02.2023 12:16, Qi Zheng wrote:
> <...>
>>>     void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>>>   {
>>> -    int i, nid;
>>> +    int i, nid, srcu_idx;
>>>       long nr;
>>>       struct mem_cgroup *parent;
>>>       struct shrinker_info *child_info, *parent_info;
>>> @@ -429,16 +443,16 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>>>           parent = root_mem_cgroup;
>>>         /* Prevent from concurrent shrinker_info expand */
>>> -    down_read(&shrinker_rwsem);
>>> +    srcu_idx = srcu_read_lock(&shrinker_srcu);
>>
>> Don't we still have to be protected against parallel expand_one_shrinker_info()?
>>
>> It looks like parent->nodeinfo[nid]->shrinker_info pointer may be changed in expand*
>> right after we've dereferenced it here.
> 
> Hi Kirill,
> 
> Oh, indeed. We may wrongly reparent the child's nr_deferred to the old
> parent's nr_deferred (it is about to be freed by call_srcu).
> 
> The reparent_shrinker_deferred() will only be called on the offline
> path (not a hotspot path), so we may be able to use shrinker_mutex
> introduced later for protection. What do you think?

It looks good for me. One more thing I'd analyzed is whether we want to have
is two reparent_shrinker_deferred() are executing in parallel.

Possible, we should leave rwsem there as it was used before..
 
>>
>>>       for_each_node(nid) {
>>> -        child_info = shrinker_info_protected(memcg, nid);
>>> -        parent_info = shrinker_info_protected(parent, nid);
>>> +        child_info = shrinker_info_srcu(memcg, nid);
>>> +        parent_info = shrinker_info_srcu(parent, nid);
>>>           for (i = 0; i < shrinker_nr_max; i++) {
>>>               nr = atomic_long_read(&child_info->nr_deferred[i]);
>>>               atomic_long_add(nr, &parent_info->nr_deferred[i]);
>>>           }
>>>       }
>>> -    up_read(&shrinker_rwsem);
>>> +    srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>   }
>>>     static bool cgroup_reclaim(struct scan_control *sc)
>>> @@ -891,15 +905,14 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>   {
>>>       struct shrinker_info *info;
>>>       unsigned long ret, freed = 0;
>>> +    int srcu_idx;
>>>       int i;
>>>         if (!mem_cgroup_online(memcg))
>>>           return 0;
>>>   -    if (!down_read_trylock(&shrinker_rwsem))
>>> -        return 0;
>>> -
>>> -    info = shrinker_info_protected(memcg, nid);
>>> +    srcu_idx = srcu_read_lock(&shrinker_srcu);
>>> +    info = shrinker_info_srcu(memcg, nid);
>>>       if (unlikely(!info))
>>>           goto unlock;
>>>   @@ -949,14 +962,9 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>                   set_shrinker_bit(memcg, nid, i);
>>>           }
>>>           freed += ret;
>>> -
>>> -        if (rwsem_is_contended(&shrinker_rwsem)) {
>>> -            freed = freed ? : 1;
>>> -            break;
>>> -        }
>>>       }
>>>   unlock:
>>> -    up_read(&shrinker_rwsem);
>>> +    srcu_read_unlock(&shrinker_srcu, srcu_idx);
>>>       return freed;
>>>   }
>>>   #else /* CONFIG_MEMCG */
>>
>