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In-Reply-To: <20210210011020.GL524633@carbon.DHCP.thefacebook.com>
From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 9 Feb 2021 17:25:16 -0800
Message-ID: <CAHbLzkpHRO1-iamUvwrg41MyzAonCPcUiObo7LFKLTbCuZptvQ@mail.gmail.com>
Subject: Re: [v7 PATCH 08/12] mm: vmscan: add per memcg shrinker nr_deferred
To:     Roman Gushchin <guro@fb.com>
Cc:     Kirill Tkhai <ktkhai@virtuozzo.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Shakeel Butt <shakeelb@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Precedence: bulk

On Tue, Feb 9, 2021 at 5:10 PM Roman Gushchin <guro@fb.com> wrote:
>
> On Tue, Feb 09, 2021 at 09:46:42AM -0800, 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 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@gmail.com>
> > ---
> >  include/linux/memcontrol.h |  7 +++---
> >  mm/vmscan.c                | 49 +++++++++++++++++++++++++-------------
> >  2 files changed, 37 insertions(+), 19 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 a047980536cf..d4b030a0b2a9 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -187,9 +187,13 @@ 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 */
> >  #define NR_MAX_TO_SHR_MAP_SIZE(nr_max) \
> >       (DIV_ROUND_UP(nr_max, BITS_PER_LONG) * sizeof(unsigned long))
> >
> > +#define NR_MAX_TO_SHR_DEF_SIZE(nr_max) \
> > +     (round_up(nr_max, BITS_PER_LONG) * sizeof(atomic_long_t))
> > +
> >  static struct shrinker_info *shrinker_info_protected(struct mem_cgroup *memcg,
> >                                                    int nid)
> >  {
> > @@ -203,10 +207,12 @@ static void free_shrinker_info_rcu(struct rcu_head *head)
> >  }
> >
> >  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 = shrinker_info_protected(memcg, nid);
> > @@ -218,9 +224,15 @@ 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->nr_deferred = (atomic_long_t *)(new + 1);
> > +             new->map = (void *)new->nr_deferred + d_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);
> > @@ -235,9 +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);
> > @@ -250,12 +259,13 @@ 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 = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
> > +     m_size = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
> > +     d_size = NR_MAX_TO_SHR_DEF_SIZE(shrinker_nr_max);
> > +     size = m_size + d_size;
> > +
> >       for_each_node(nid) {
> >               info = kvzalloc_node(sizeof(*info) + size, GFP_KERNEL, nid);
> >               if (!info) {
> > @@ -263,6 +273,8 @@ 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 + d_size;
> >               rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> >       }
> >       up_write(&shrinker_rwsem);
> > @@ -274,10 +286,16 @@ 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 = NR_MAX_TO_SHR_MAP_SIZE(new_nr_max);
> > -     old_size = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
> > +     m_size = NR_MAX_TO_SHR_MAP_SIZE(new_nr_max);
> > +     d_size = NR_MAX_TO_SHR_DEF_SIZE(new_nr_max);
> > +     size = m_size + d_size;
> > +     old_m_size = NR_MAX_TO_SHR_MAP_SIZE(shrinker_nr_max);
> > +     old_d_size = NR_MAX_TO_SHR_DEF_SIZE(shrinker_nr_max);
> > +     old_size = old_m_size + old_d_size;
> >       if (size <= old_size)
> >               goto out;
>
> It looks correct, but a bit bulky. Can we check that the new maximum
> number of elements is larger than then the old one here?

Seems not to me. For example, we have shrinker_nr_max as 1, then a new
shrinker is registered and the new_nr_max is 2, but actually the new
size is equal to the old size.

We should be able to do:
if (round_up(new_nr_max, BITS_PER_LONG) <= round_up(shrinker_nr_mx,
BITS_PER_LONG))

Does it seem better?

>
> >
> > @@ -286,9 +304,8 @@ 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);
>
> Pass the old and the new numbers to expand_one_shrinker_info() and
> have all size manipulation there?

With the above proposal we could move the size manipulation right
before the memcg iter, we could save some cycles if we don't have to
expand it.

>
> >               if (ret) {
> >                       mem_cgroup_iter_break(NULL, memcg);
> >                       goto out;
> > --
> > 2.26.2
> >