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@xxxxxxxxx>
>> ---
>> 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;
>>
>
