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Message-ID: <CAHbLzko_1VydJHurX4fACw4v9v859dUbCwSpvhBOnDoKiwu0pQ@mail.gmail.com>
Date: Wed, 13 Jan 2021 15:30:22 -0800
From: Yang Shi <shy828301@...il.com>
To: Kirill Tkhai <ktkhai@...tuozzo.com>
Cc: Roman Gushchin <guro@...com>, Shakeel Butt <shakeelb@...gle.com>,
Dave Chinner <david@...morbit.com>,
Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...e.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux MM <linux-mm@...ck.org>,
Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v3 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred
On Wed, Jan 6, 2021 at 3:07 AM Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
>
> On 06.01.2021 01:58, 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@...il.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 e05bbe8277cc..5599082df623 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 memcg_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 0033659abf9e..72259253e414 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -193,10 +193,12 @@ static void memcg_free_shrinker_info_rcu(struct rcu_head *head)
> > }
> >
> > static int memcg_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 memcg_shrinker_info *new, *old;
> > int nid;
> > + int size = m_size + d_size;
> >
> > for_each_node(nid) {
> > old = rcu_dereference_protected(
> > @@ -209,9 +211,18 @@ static int memcg_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 *)((unsigned long)new + sizeof(*new));
> > + new->nr_deferred = (atomic_long_t *)((unsigned long)new +
> > + sizeof(*new) + m_size);
>
> Can't we write this more compact?
>
> new->map = (unsigned long *)(new + 1);
> new->nr_deferred = (atomic_long_t)(new->map + 1);
By relooking this, the second line looks wrong. The layout should be:
----------------------------
| struct shrinker_info |
-----------------------------
| map array |
-----------------------------
| nr_deferred array |
------------------------------
new->map is the pointer to map array, its type is "unsigned long *",
so "new->map + 1" should point to the next 32 bytes, but the map array
may occupy more than one "unsigned long", this would corrupt the
arrays.
I think we could use "new->map + (shrinker_nr_max / BITS_PER_LONG) + 1"
>
> > +
> > + /* 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((void *)new->nr_deferred, (void *)old->nr_deferred,
> > + old_d_size);
>
> Why not
> 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, memcg_free_shrinker_info_rcu);
> > @@ -226,9 +237,6 @@ void memcg_free_shrinker_info(struct mem_cgroup *memcg)
> > struct memcg_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 memcg_alloc_shrinker_info(struct mem_cgroup *memcg)
> > {
> > struct memcg_shrinker_info *info;
> > int nid, size, ret = 0;
> > -
> > - if (mem_cgroup_is_root(memcg))
> > - return 0;
> > + int m_size, d_size = 0;
> >
> > down_read(&shrinker_rwsem);
> > - size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > + m_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > + d_size = shrinker_nr_max * sizeof(atomic_long_t);
> > + size = m_size + d_size;
> > +
> > for_each_node(nid) {
> > info = kvzalloc(sizeof(*info) + size, GFP_KERNEL);
> > if (!info) {
> > @@ -255,6 +264,9 @@ int memcg_alloc_shrinker_info(struct mem_cgroup *memcg)
> > ret = -ENOMEM;
> > break;
> > }
> > + info->map = (unsigned long *)((unsigned long)info + sizeof(*info));
> > + info->nr_deferred = (atomic_long_t *)((unsigned long)info +
> > + sizeof(*info) + m_size);
> > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_info, info);
> > }
> > up_read(&shrinker_rwsem);
> > @@ -265,10 +277,16 @@ int memcg_alloc_shrinker_info(struct mem_cgroup *memcg)
> > static int memcg_expand_shrinker_info(int new_id)
> > {
> > int size, old_size, ret = 0;
> > + int m_size, d_size = 0;
> > + int old_m_size, old_d_size = 0;
> > struct mem_cgroup *memcg;
> >
> > - size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> > - old_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned long);
> > + m_size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> > + d_size = (new_id + 1) * sizeof(atomic_long_t);
> > + size = m_size + d_size;
> > + old_m_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * 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;
>
> This replication of patch [4/11] looks awkwardly. Please, try to incorporate
> the same changes to nr_deferred as I requested for shrinker_map in [4/11].
>
> >
> > @@ -277,9 +295,8 @@ static int memcg_expand_shrinker_info(int new_id)
> >
> > memcg = mem_cgroup_iter(NULL, NULL, NULL);
> > do {
> > - if (mem_cgroup_is_root(memcg))
> > - continue;
> > - ret = memcg_expand_one_shrinker_info(memcg, size, old_size);
> > + ret = memcg_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;
> >
>
>
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