On Mon, Mar 8, 2021 at 11:12 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Tue, Feb 16, 2021 at 4:13 PM Yang Shi <shy828301@xxxxxxxxx> 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 ratio 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 | 60 ++++++++++++++++++++++++++------------
> > 2 files changed, 46 insertions(+), 21 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 4c9253896e25..c457fc7bc631 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[];
> > + atomic_long_t *nr_deferred;
> > + unsigned long *map;
> > };
> >
> > /*
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index a1047ea60ecf..fcb399e18fc3 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -187,11 +187,17 @@ static DECLARE_RWSEM(shrinker_rwsem);
> > #ifdef CONFIG_MEMCG
> > static int shrinker_nr_max;
> >
> > +/* The shrinker_info is expanded in a batch of BITS_PER_LONG */
> > static inline int shrinker_map_size(int nr_items)
> > {
> > return (DIV_ROUND_UP(nr_items, BITS_PER_LONG) * sizeof(unsigned long));
> > }
> >
> > +static inline int shrinker_defer_size(int nr_items)
> > +{
> > + return (round_up(nr_items, BITS_PER_LONG) * sizeof(atomic_long_t));
> > +}
> > +
> > static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
> > int nid)
> > {
> > @@ -200,10 +206,12 @@ static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
> > }
> >
> > static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> > - int size, int old_size)
> > + int map_size, int defer_size,
> > + int old_map_size, int old_defer_size)
> > {
> > struct shrinker_info *new, *old;
> > int nid;
> > + int size = map_size + defer_size;
> >
> > for_each_node(nid) {
> > old = shrinker_info_protected(memcg, nid);
> > @@ -215,9 +223,16 @@ 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->nr_deferred = (atomic_long_t *)(new + 1);
> > + new->map = (void *)new->nr_deferred + defer_size;
> > +
> > + /* map: set all old bits, clear all new bits */
> > + memset(new->map, (int)0xff, old_map_size);
> > + memset((void *)new->map + old_map_size, 0, map_size - old_map_size);
> > + /* nr_deferred: copy old values, clear all new values */
> > + memcpy(new->nr_deferred, old->nr_deferred, old_defer_size);
> > + memset((void *)new->nr_deferred + old_defer_size, 0,
> > + defer_size - old_defer_size);
> >
> > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, new);
> > kvfree_rcu(old);
> > @@ -232,9 +247,6 @@ 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 = shrinker_info_protected(memcg, nid);
> > @@ -247,12 +259,12 @@ 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;
>
> Can you please comment on the consequences on allowing to allocate
> shrinker_info for root memcg? Why didn't we do that before but now it
> is fine (or maybe required)? Please add the explanation in the commit
> message.
Before the patchset shrinker_info just tracks shrinker_maps which is
not required for root memcg. But the newly added nr_deferred is needed
in root memcg otherwise the nr_deferred work would get lost once the
memcgs are reparented to root.
How's about adding the below paragraph to the commit log:
"To preserve nr_deferred when reparenting memcgs to root, root memcg
needs shrinker_info allocated too."
>
> > + int map_size, defer_size = 0;
> >
> > down_write(&shrinker_rwsem);
> > - size = shrinker_map_size(shrinker_nr_max);
> > + map_size = shrinker_map_size(shrinker_nr_max);
> > + defer_size = shrinker_defer_size(shrinker_nr_max);
> > + size = map_size + defer_size;
> > for_each_node(nid) {
> > info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
> > if (!info) {
> > @@ -260,6 +272,8 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
> > ret = -ENOMEM;
> > break;
> > }
> > + info->nr_deferred = (atomic_long_t *)(info + 1);
> > + info->map = (void *)info->nr_deferred + defer_size;
> > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> > }
> > up_write(&shrinker_rwsem);
> > @@ -267,15 +281,21 @@ 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 size, old_size, ret = 0;
> > + int ret = 0;
> > int new_nr_max = new_id + 1;
> > + int map_size, defer_size = 0;
> > + int old_map_size, old_defer_size = 0;
> > struct mem_cgroup *memcg;
> >
> > - size = shrinker_map_size(new_nr_max);
> > - old_size = shrinker_map_size(shrinker_nr_max);
> > - if (size <= old_size)
> > + if (!need_expand(new_nr_max))
> > goto out;
> >
> > if (!root_mem_cgroup)
> > @@ -283,11 +303,15 @@ static int expand_shrinker_info(int new_id)
> >
> > lockdep_assert_held(&shrinker_rwsem);
> >
> > + map_size = shrinker_map_size(new_nr_max);
> > + defer_size = shrinker_defer_size(new_nr_max);
> > + old_map_size = shrinker_map_size(shrinker_nr_max);
> > + old_defer_size = shrinker_defer_size(shrinker_nr_max);
> > +
> > 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, map_size, defer_size,
> > + old_map_size, old_defer_size);
> > if (ret) {
> > mem_cgroup_iter_break(NULL, memcg);
> > goto out;
> > --
> > 2.26.2
> >
