Message-ID: <d018f3b3-2f81-0a17-8e4d-fa5aad59e196@bytedance.com>
Date:   Sun, 26 Feb 2023 21:54:53 +0800
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Subject: Re: [PATCH v2 1/7] mm: vmscan: add a map_nr_max field to
 shrinker_info
Content-Language: en-US
To:     Kirill Tkhai <tkhai@ya.ru>
Cc:     sultan@kerneltoast.com, dave@stgolabs.net,
        penguin-kernel@I-love.SAKURA.ne.jp, paulmck@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        David Hildenbrand <david@redhat.com>,
        Yang Shi <shy828301@gmail.com>
References: <20230223132725.11685-1-zhengqi.arch@bytedance.com>
 <20230223132725.11685-2-zhengqi.arch@bytedance.com>
 <6f8f01b5-d802-db64-7725-8481c67c13a2@bytedance.com>
 <ea1c5d49-1efa-cce8-8750-e19c56187a7c@ya.ru>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <ea1c5d49-1efa-cce8-8750-e19c56187a7c@ya.ru>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk


On 2023/2/25 23:14, Kirill Tkhai wrote:
> Hi Qi,
> 
> On 25.02.2023 11:18, Qi Zheng wrote:
>>
>>
>> On 2023/2/23 21:27, Qi Zheng wrote:
>>> To prepare for the subsequent lockless memcg slab shrink,
>>> add a map_nr_max field to struct shrinker_info to records
>>> its own real shrinker_nr_max.
>>>
>>> No functional changes.
>>>
>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
>>
>> I missed Suggested-by here, hi Kirill, can I add it?
>>
>> Suggested-by: Kirill Tkhai <tkhai@ya.ru>
> 
> Yes, feel free to add this tag.
> 
> There is a comment below.
> 
>>> ---
>>>    include/linux/memcontrol.h |  1 +
>>>    mm/vmscan.c                | 29 ++++++++++++++++++-----------
>>>    2 files changed, 19 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>>> index b6eda2ab205d..aa69ea98e2d8 100644
>>> --- a/include/linux/memcontrol.h
>>> +++ b/include/linux/memcontrol.h
>>> @@ -97,6 +97,7 @@ struct shrinker_info {
>>>        struct rcu_head rcu;
>>>        atomic_long_t *nr_deferred;
>>>        unsigned long *map;
>>> +    int map_nr_max;
>>>    };
>>>      struct lruvec_stats_percpu {
>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>> index 9c1c5e8b24b8..9f895ca6216c 100644
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -224,9 +224,16 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>>>                         lockdep_is_held(&shrinker_rwsem));
>>>    }
>>>    +static inline bool need_expand(int new_nr_max, int old_nr_max)
>>> +{
>>> +    return round_up(new_nr_max, BITS_PER_LONG) >
>>> +           round_up(old_nr_max, BITS_PER_LONG);
>>> +}
>>> +
>>>    static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>>                        int map_size, int defer_size,
>>> -                    int old_map_size, int old_defer_size)
>>> +                    int old_map_size, int old_defer_size,
>>> +                    int new_nr_max)
>>>    {
>>>        struct shrinker_info *new, *old;
>>>        struct mem_cgroup_per_node *pn;
>>> @@ -240,12 +247,16 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>>            if (!old)
>>>                return 0;
>>>    +        if (!need_expand(new_nr_max, old->map_nr_max))
>>> +            return 0;
>>> +
>>>            new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
>>>            if (!new)
>>>                return -ENOMEM;
>>>              new->nr_deferred = (atomic_long_t *)(new + 1);
>>>            new->map = (void *)new->nr_deferred + defer_size;
>>> +        new->map_nr_max = new_nr_max;
>>>              /* map: set all old bits, clear all new bits */
>>>            memset(new->map, (int)0xff, old_map_size);
>>> @@ -295,6 +306,7 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>            }
>>>            info->nr_deferred = (atomic_long_t *)(info + 1);
>>>            info->map = (void *)info->nr_deferred + defer_size;
>>> +        info->map_nr_max = shrinker_nr_max;
>>>            rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>>        }
>>>        up_write(&shrinker_rwsem);
>>> @@ -302,12 +314,6 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>>        return ret;
>>>    }
>>>    -static inline bool need_expand(int nr_max)
>>> -{
>>> -    return round_up(nr_max, BITS_PER_LONG) >
>>> -           round_up(shrinker_nr_max, BITS_PER_LONG);
>>> -}
>>> -
>>>    static int expand_shrinker_info(int new_id)
>>>    {
>>>        int ret = 0;
>>> @@ -316,7 +322,7 @@ static int expand_shrinker_info(int new_id)
>>>        int old_map_size, old_defer_size = 0;
>>>        struct mem_cgroup *memcg;
>>>    -    if (!need_expand(new_nr_max))
>>> +    if (!need_expand(new_nr_max, shrinker_nr_max))
>>>            goto out;
>>>          if (!root_mem_cgroup)
>>> @@ -332,7 +338,8 @@ static int expand_shrinker_info(int new_id)
>>>        memcg = mem_cgroup_iter(NULL, NULL, NULL);
>>>        do {
>>>            ret = expand_one_shrinker_info(memcg, map_size, defer_size,
>>> -                           old_map_size, old_defer_size);
>>> +                           old_map_size, old_defer_size,
>>> +                           new_nr_max);
>>>            if (ret) {
>>>                mem_cgroup_iter_break(NULL, memcg);
>>>                goto out;
>>> @@ -432,7 +439,7 @@ void reparent_shrinker_deferred(struct mem_cgroup *memcg)
>>>        for_each_node(nid) {
>>>            child_info = shrinker_info_protected(memcg, nid);
>>>            parent_info = shrinker_info_protected(parent, nid);
>>> -        for (i = 0; i < shrinker_nr_max; i++) {
>>> +        for (i = 0; i < child_info->map_nr_max; i++) {
>>>                nr = atomic_long_read(&child_info->nr_deferred[i]);
>>>                atomic_long_add(nr, &parent_info->nr_deferred[i]);
>>>            }
>>> @@ -899,7 +906,7 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid,
>>>        if (unlikely(!info))
>>>            goto unlock;
>>>    -    for_each_set_bit(i, info->map, shrinker_nr_max) {
>>> +    for_each_set_bit(i, info->map, info->map_nr_max) {
>>>            struct shrink_control sc = {
>>>                .gfp_mask = gfp_mask,
>>>                .nid = nid,
> 
> The patch as whole thing won't work as expected. It won't ever call shrinker with ids from [round_down(shrinker_nr_max, sizeof(unsigned long)) + 1, shrinker_nr_max - 1]
> 
> Just replay the sequence we add new shrinkers:
> 
> 1)We add shrinker #0:
>     shrinker_nr_max = 0;
> 
>     prealloc_memcg_shrinker()
>        id = 0;
>        expand_shrinker_info(0)
>          new_nr_max = 1;
>          expand_one_shrinker_info(new_nr_max = 1)
>            new->map_nr_max = 1;
>          shrinker_nr_max = 1;
> 
> 2)We add shrinker #1:
>     prealloc_memcg_shrinker()
>       id = 1;
>       expand_shrinker_info(1)
>         new_nr_max = 2;
>         need_expand(2, 1) => false => ignore expand
>         shrinker_nr_max = 2;
> 
> 3)Then we call shrinker:
>    shrink_slab_memcg()
>      for_each_set_bit(i, info->map, 1/* info->map_nr_max */ ) {
>      } => ignore shrinker #1
> 
> I'd fixed this patch by something like the below:
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 9f895ca6216c..bb617a3871f1 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -224,12 +224,6 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
>   					 lockdep_is_held(&shrinker_rwsem));
>   }
>   
> -static inline bool need_expand(int new_nr_max, int old_nr_max)
> -{
> -	return round_up(new_nr_max, BITS_PER_LONG) >
> -	       round_up(old_nr_max, BITS_PER_LONG);
> -}
> -
>   static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>   				    int map_size, int defer_size,
>   				    int old_map_size, int old_defer_size,
> @@ -247,9 +241,6 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>   		if (!old)
>   			return 0;
>   
> -		if (!need_expand(new_nr_max, old->map_nr_max))
> -			return 0;
> -

Maybe we can keep this. For example, when we failed to allocate memory 
by calling kvmalloc_node() last time, some shrinker_info may have been
expanded, and these shrinker_info do not need to be expanded again.

>   		new = kvmalloc_node(sizeof(*new) + size, GFP_KERNEL, nid);
>   		if (!new)
>   			return -ENOMEM;
> @@ -317,14 +308,11 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>   static int expand_shrinker_info(int new_id)
>   {
>   	int ret = 0;
> -	int new_nr_max = new_id + 1;
> +	int new_nr_max = round_up(new_id + 1, BITS_PER_LONG);
>   	int map_size, defer_size = 0;
>   	int old_map_size, old_defer_size = 0;
>   	struct mem_cgroup *memcg;
>   
> -	if (!need_expand(new_nr_max, shrinker_nr_max))
> -		goto out;
> -
>   	if (!root_mem_cgroup)
>   		goto out;
>   
> @@ -359,9 +347,11 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
>   
>   		rcu_read_lock();
>   		info = rcu_dereference(memcg->nodeinfo[nid]->shrinker_info);
> -		/* Pairs with smp mb in shrink_slab() */
> -		smp_mb__before_atomic();
> -		set_bit(shrinker_id, info->map);
> +		if (!WARN_ON_ONCE(shrinker_id >= info->map_nr_max)) {
> +			/* Pairs with smp mb in shrink_slab() */
> +			smp_mb__before_atomic();
> +			set_bit(shrinker_id, info->map);
> +		}
>   		rcu_read_unlock();
>   	}
>   }
> 
> (I also added a new check into set_shrinker_bit() for safety).
> 
> Kirill

-- 
Thanks,
Qi