Re: [v5 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 29.01.2021 16:00, Vlastimil Babka wrote:
> On 1/28/21 12:33 AM, Yang Shi wrote:
>> Currently the number of deferred objects are per shrinker, but some slabs, for example,
>> vfs inode/dentry cache are per memcg, this would result in poor isolation among memcgs.
>>
>> The deferred objects typically are generated by __GFP_NOFS allocations, one memcg with
>> excessive __GFP_NOFS allocations may blow up deferred objects, then other innocent memcgs
>> may suffer from over shrink, excessive reclaim latency, etc.
>>
>> For example, two workloads run in memcgA and memcgB respectively, workload in B is vfs
>> heavy workload.  Workload in A generates excessive deferred objects, then B's vfs cache
>> might be hit heavily (drop half of caches) by B's limit reclaim or global reclaim.
>>
>> We observed this hit in our production environment which was running vfs heavy workload
>> shown as the below tracing log:
>>
>> <...>-409454 [016] .... 28286961.747146: mm_shrink_slab_start: super_cache_scan+0x0/0x1a0 ffff9a83046f3458:
>> nid: 1 objects to shrink 3641681686040 gfp_flags GFP_HIGHUSER_MOVABLE|__GFP_ZERO pgs_scanned 1 lru_pgs 15721
>> cache items 246404277 delta 31345 total_scan 123202138
>> <...>-409454 [022] .... 28287105.928018: mm_shrink_slab_end: super_cache_scan+0x0/0x1a0 ffff9a83046f3458:
>> nid: 1 unused scan count 3641681686040 new scan count 3641798379189 total_scan 602
>> last shrinker return val 123186855
>>
>> The vfs cache and page cache ration was 10:1 on this machine, and half of caches were dropped.
>> This also resulted in significant amount of page caches were dropped due to inodes eviction.
>>
>> Make nr_deferred per memcg for memcg aware shrinkers would solve the unfairness and bring
>> better isolation.
>>
>> When memcg is not enabled (!CONFIG_MEMCG or memcg disabled), the shrinker's nr_deferred
>> would be used.  And non memcg aware shrinkers use shrinker's nr_deferred all the time.
>>
>> Signed-off-by: Yang Shi <shy828301@gmail.com>
>> ---
>>  include/linux/memcontrol.h |  7 +++---
>>  mm/vmscan.c                | 48 +++++++++++++++++++++++++-------------
>>  2 files changed, 36 insertions(+), 19 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 62b888b88a5f..e0384367e07d 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -93,12 +93,13 @@ struct lruvec_stat {
>>  };
>>  
>>  /*
>> - * Bitmap of shrinker::id corresponding to memcg-aware shrinkers,
>> - * which have elements charged to this memcg.
>> + * Bitmap and deferred work of shrinker::id corresponding to memcg-aware
>> + * shrinkers, which have elements charged to this memcg.
>>   */
>>  struct shrinker_info {
>>  	struct rcu_head rcu;
>> -	unsigned long map[];
>> +	unsigned long *map;
>> +	atomic_long_t *nr_deferred;
>>  };
>>  
>>  /*
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 256896d157d4..20be0db291fe 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -187,16 +187,21 @@ static DECLARE_RWSEM(shrinker_rwsem);
>>  #ifdef CONFIG_MEMCG
>>  static int shrinker_nr_max;
>>  
>> +#define NR_MAX_TO_SHR_MAP_SIZE(nr_max)	\
>> +	((nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long))
> 
> Could have been part of patch 4 already. And yeah, using DIV_ROUND_UP(), as
> being hidden in a macro makes the "shorter statement" benefit disappear :)

This was requested by me, and I am agree with Vlastimil using DIV_ROUND_UP()
is OK.

>> +
>>  static void free_shrinker_info_rcu(struct rcu_head *head)
>>  {
>>  	kvfree(container_of(head, struct shrinker_info, rcu));
>>  }
>>  
>>  static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>> -				   int size, int old_size)
>> +				    int m_size, int d_size,
>> +				    int old_m_size, int old_d_size)
>>  {
>>  	struct shrinker_info *new, *old;
>>  	int nid;
>> +	int size = m_size + d_size;
>>  
>>  	for_each_node(nid) {
>>  		old = rcu_dereference_protected(
>> @@ -209,9 +214,15 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
>>  		if (!new)
>>  			return -ENOMEM;
>>  
>> -		/* Set all old bits, clear all new bits */
>> -		memset(new->map, (int)0xff, old_size);
>> -		memset((void *)new->map + old_size, 0, size - old_size);
>> +		new->map = (unsigned long *)(new + 1);
>> +		new->nr_deferred = (void *)new->map + m_size;
> 
> This better be aligned to sizeof(atomic_long_t). Can we be sure about that?
> Also it's all quite ugly and complex. Is it worth it? What about just leaving
> map as it is and allocating a nr_deferred array separately, i.e.:
>
>   struct shrinker_info {
>  	struct rcu_head rcu;
> 	atomic_long_t *nr_deferred; // allocated separately
> 	unsigned long map[];
>   };

Hm, don't two kvmalloc()'ed areas placed together occupy less memory?!
nr_deferred easily can occupy n pages + small tail bytes since 4096/8
is just 512 shrinkers.
 
>> +		/* map: set all old bits, clear all new bits */
>> +		memset(new->map, (int)0xff, old_m_size);
>> +		memset((void *)new->map + old_m_size, 0, m_size - old_m_size);
>> +		/* nr_deferred: copy old values, clear all new values */
>> +		memcpy(new->nr_deferred, old->nr_deferred, old_d_size);
>> +		memset((void *)new->nr_deferred + old_d_size, 0, d_size - old_d_size);
>>  
>>  		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, new);
>>  		call_rcu(&old->rcu, free_shrinker_info_rcu);
>> @@ -226,9 +237,6 @@ void free_shrinker_info(struct mem_cgroup *memcg)
>>  	struct shrinker_info *info;
>>  	int nid;
>>  
>> -	if (mem_cgroup_is_root(memcg))
>> -		return;
>> -
>>  	for_each_node(nid) {
>>  		pn = mem_cgroup_nodeinfo(memcg, nid);
>>  		info = rcu_dereference_protected(pn->shrinker_info, true);
>> @@ -242,12 +250,13 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>  {
>>  	struct shrinker_info *info;
>>  	int nid, size, ret = 0;
>> -
>> -	if (mem_cgroup_is_root(memcg))
>> -		return 0;
>> +	int m_size, d_size = 0;
>>  
>>  	down_write(&shrinker_rwsem);
>> -	size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>> +	m_size = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
>> +	d_size = shrinker_nr_max * sizeof(atomic_long_t);
>> +	size = m_size + d_size;
>> +
>>  	for_each_node(nid) {
>>  		info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
>>  		if (!info) {
>> @@ -255,6 +264,8 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
>>  			ret = -ENOMEM;
>>  			break;
>>  		}
>> +		info->map = (unsigned long *)(info + 1);
>> +		info->nr_deferred = (void *)info->map + m_size;
>>  		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
>>  	}
>>  	up_write(&shrinker_rwsem);
>> @@ -266,10 +277,16 @@ static int expand_shrinker_info(int new_id)
>>  {
>>  	int size, old_size, ret = 0;
>>  	int new_nr_max = new_id + 1;
>> +	int m_size, d_size = 0;
>> +	int old_m_size, old_d_size = 0;
>>  	struct mem_cgroup *memcg;
>>  
>> -	size = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>> -	old_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
>> +	m_size = NR_MAX_TO_SHR_MAP_SIZE(new_nr_max);
>> +	d_size = new_nr_max * sizeof(atomic_long_t);
>> +	size = m_size + d_size;
>> +	old_m_size = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
>> +	old_d_size = shrinker_nr_max * sizeof(atomic_long_t);
>> +	old_size = old_m_size + old_d_size;
>>  	if (size <= old_size)
>>  		goto out;
>>  
>> @@ -278,9 +295,8 @@ static int expand_shrinker_info(int new_id)
>>  
>>  	memcg = mem_cgroup_iter(NULL, NULL, NULL);
>>  	do {
>> -		if (mem_cgroup_is_root(memcg))
>> -			continue;
>> -		ret = expand_one_shrinker_info(memcg, size, old_size);
>> +		ret = expand_one_shrinker_info(memcg, m_size, d_size,
>> +					       old_m_size, old_d_size);
>>  		if (ret) {
>>  			mem_cgroup_iter_break(NULL, memcg);
>>  			goto out;
>>
>