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    SubjectRe: [v4 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred
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    On 22.01.2021 02:06, 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 | 49 +++++++++++++++++++++++++-------------
    > 2 files changed, 36 insertions(+), 20 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 018e1beb24c9..722aa71b13b2 100644
    > --- a/mm/vmscan.c
    > +++ b/mm/vmscan.c
    > @@ -192,11 +192,13 @@ 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)
    > +static int expand_one_shrinker_info(struct mem_cgroup *memcg, int nr_max,
    > + 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 +211,16 @@ 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 = (atomic_long_t *)(new->map +
    > + nr_max / BITS_PER_LONG + 1);

    Why not

    new->nr_deferred = (void *)new->map + m_size;
    ?

    > +
    > + /* 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 +235,6 @@ 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 +248,13 @@ 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 = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
    > + 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 +262,9 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
    > ret = -ENOMEM;
    > break;
    > }
    > + info->map = (unsigned long *)(info + 1);
    > + info->nr_deferred = (atomic_long_t *)(info->map +
    > + shrinker_nr_max / BITS_PER_LONG + 1);

    Why not:
    info->nr_deferred = (void*)info->map + m_size;
    ?

    > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
    > }
    > up_write(&shrinker_rwsem);
    > @@ -266,10 +276,16 @@ 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 = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
    > + d_size = new_nr_max * sizeof(atomic_long_t);
    > + size = m_size + d_size;
    > + old_m_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);

    Could you please pack this twice repeating pattern into some macro? E.g.,

    #define NR_MAX_TO_SHR_MAP_SIZE(nr_max) \
    ((nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long))

    > + old_d_size = shrinker_nr_max * sizeof(atomic_long_t);
    > + old_size = old_m_size + old_d_size;
    > if (size <= old_size)
    > return 0;
    >
    > @@ -278,9 +294,8 @@ 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, new_nr_max, m_size, d_size,
    > + old_m_size, old_d_size);
    > if (ret) {
    > mem_cgroup_iter_break(NULL, memcg);
    > goto out;
    >

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