The shrinker_rwsem is a global read-write lock in shrinkers subsystem,
which protects most operations such as slab shrink, registration and
unregistration of shrinkers, etc. This can easily cause problems in the
following cases.

1) When the memory pressure is high and there are many filesystems
   mounted or unmounted at the same time, slab shrink will be affected
   (down_read_trylock() failed).

   Such as the real workload mentioned by Kirill Tkhai:

   ```
   One of the real workloads from my experience is start
   of an overcommitted node containing many starting
   containers after node crash (or many resuming containers
   after reboot for kernel update). In these cases memory
   pressure is huge, and the node goes round in long reclaim.
   ```

2) If a shrinker is blocked (such as the case mentioned
   in [1]) and a writer comes in (such as mount a fs),
   then this writer will be blocked and cause all
   subsequent shrinker-related operations to be blocked.

Even if there is no competitor when shrinking slab, there may still be a
problem. The down_read_trylock() may become a perf hotspot with frequent
calls to shrink_slab(). Because of the poor multicore scalability of
atomic operations, this can lead to a significant drop in IPC
(instructions per cycle).

We used to implement the lockless slab shrink with SRCU [2], but then
kernel test robot reported -88.8% regression in
stress-ng.ramfs.ops_per_sec test case [3], so we reverted it [4].

This commit uses the refcount+RCU method [5] proposed by Dave Chinner
to re-implement the lockless global slab shrink. The memcg slab shrink is
handled in the subsequent patch.

For now, all shrinker instances are converted to dynamically allocated and
will be freed by call_rcu(). So we can use rcu_read_{lock,unlock}() to
ensure that the shrinker instance is valid.

And the shrinker instance will not be run again after unregistration. So
the structure that records the pointer of shrinker instance can be safely
freed without waiting for the RCU read-side critical section.

In this way, while we implement the lockless slab shrink, we don't need to
be blocked in unregister_shrinker().

The following are the test results:

stress-ng --timeout 60 --times --verify --metrics-brief --ramfs 9 &

1) Before applying this patchset:

setting to a 60 second run per stressor
dispatching hogs: 9 ramfs
stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
                          (secs)    (secs)    (secs)   (real time) (usr+sys time)
ramfs            473062     60.00      8.00    279.13      7884.12        1647.59
for a 60.01s run time:
   1440.34s available CPU time
      7.99s user time   (  0.55%)
    279.13s system time ( 19.38%)
    287.12s total time  ( 19.93%)
load average: 7.12 2.99 1.15
successful run completed in 60.01s (1 min, 0.01 secs)

2) After applying this patchset:

setting to a 60 second run per stressor
dispatching hogs: 9 ramfs
stressor       bogo ops real time  usr time  sys time   bogo ops/s     bogo ops/s
                          (secs)    (secs)    (secs)   (real time) (usr+sys time)
ramfs            477165     60.00      8.13    281.34      7952.55        1648.40
for a 60.01s run time:
   1440.33s available CPU time
      8.12s user time   (  0.56%)
    281.34s system time ( 19.53%)
    289.46s total time  ( 20.10%)
load average: 6.98 3.03 1.19
successful run completed in 60.01s (1 min, 0.01 secs)

We can see that the ops/s has hardly changed.

[1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/
[2]. https://lore.kernel.org/lkml/20230313112819.38938-1-zhengqi.arch@bytedance.com/
[3]. https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@intel.com/
[4]. https://lore.kernel.org/all/20230609081518.3039120-1-qi.zheng@linux.dev/
[5]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@dread.disaster.area/

Link: https://lkml.kernel.org/r/20230911094444.68966-43-zhengqi.arch@bytedance.comSigned-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bob Peterson <rpeterso@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Chandan Babu R <chandan.babu@oracle.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Chris Mason <clm@fb.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Chuck Lever <cel@kernel.org>
Cc: Coly Li <colyli@suse.de>
Cc: Dai Ngo <Dai.Ngo@oracle.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Airlie <airlied@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Sterba <dsterba@suse.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Gao Xiang <hsiangkao@linux.alibaba.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Jeffle Xu <jefflexu@linux.alibaba.com>
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Kirill Tkhai <tkhai@ya.ru>
Cc: Marijn Suijten <marijn.suijten@somainline.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Nadav Amit <namit@vmware.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Cc: Olga Kornievskaia <kolga@netapp.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Richard Weinberger <richard@nod.at>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Sean Paul <sean@poorly.run>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Song Liu <song@kernel.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Yue Hu <huyue2@coolpad.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>