[PATCH RFC 00/10] Introduce lockless shrink_slab()

Kirill Tkhai
2018-08-07T15:37:46Z

After bitmaps of not-empty memcg shrinkers were implemented
(see "[PATCH v9 00/17] Improve shrink_slab() scalability..."
series, which is already in mm tree), all the evil in perf
trace has moved from shrink_slab() to down_read_trylock().
As reported by Shakeel Butt:

> I created 255 memcgs, 255 ext4 mounts and made each memcg create a
> file containing few KiBs on corresponding mount. Then in a separate
> memcg of 200 MiB limit ran a fork-bomb.
>
> I ran the "perf record -ag -- sleep 60" and below are the results:
> + 47.49% fb.sh [kernel.kallsyms] [k] down_read_trylock
> + 30.72% fb.sh [kernel.kallsyms] [k] up_read
> + 9.51% fb.sh [kernel.kallsyms] [k] mem_cgroup_iter
> + 1.69% fb.sh [kernel.kallsyms] [k] shrink_node_memcg
> + 1.35% fb.sh [kernel.kallsyms] [k] mem_cgroup_protected
> + 1.05% fb.sh [kernel.kallsyms] [k] queued_spin_lock_slowpath
> + 0.85% fb.sh [kernel.kallsyms] [k] _raw_spin_lock
> + 0.78% fb.sh [kernel.kallsyms] [k] lruvec_lru_size
> + 0.57% fb.sh [kernel.kallsyms] [k] shrink_node
> + 0.54% fb.sh [kernel.kallsyms] [k] queue_work_on
> + 0.46% fb.sh [kernel.kallsyms] [k] shrink_slab_memcg

The patchset continues to improve shrink_slab() scalability and makes
it lockless completely. Here are several steps for that:

1)Use SRCU to synchronize shrink_slab() and unregister_shrinker().
Nothing exiting is here, just srcu_read_lock() in shrink_slab()
and shrink_slab_memcg() and synchronize_srcu() in unregister_shrinker().
See [2/10] for details.
2)The above requires to make SRCU unconditional enabled.
[1/10] makes this. Note, that if we can't always enable
SRCU, we may use percpu_rw_semaphore instead of this.
See comment to [2/10] for details.
3)Convert shrinker_rwsem to mutex. Just cleanup.

4)Further patches make possible to speed up unregister_shrinker()
by splitting it in two stages. The first stage unlinks shrinker
from shrinker_list and shrinker_idr, while the second finalizes
the thing by calling synchronize_srcu() and freeing memory.
Patch [4/10] actually splits unregister_shrinker(), while
[10/10] makes superblock shrinker to use the new helpers
(unregister_shrinker_delayed_{initiate,finalize}().

5)Patches [5-9/10] are preparations on fs, which make possible
to split superblock unregistration in two stages. They sequentially
make super_cache_count() and super_cache_scan() safe to be called
on unregistering shrinker:

[cpu0] [cpu1]
unregister_shrinker_delayed_initiate(shrinker)
... shrink_slab(shrinker) (OK!)
unregister_shrinker_delayed_finalize(shrinker)

After all of this, shrink_slab() becomes lockless, while unregister_shrinker()
remains fast at least for superblock shrinker (another shrinkers also
can be made to unregister in the same delayed manner).

(This requires "mm: Use special value SHRINKER_REGISTERING instead list_empty() check"
from https://lkml.org/lkml/2018/8/6/276, which is on the way to -mm tree, as said
by -mm tree notification message from Andrew).

---

Kirill Tkhai (10):
rcu: Make CONFIG_SRCU unconditionally enabled
mm: Make shrink_slab() lockless
mm: Convert shrinker_rwsem to mutex
mm: Split unregister_shrinker()
fs: Move list_lru_destroy() to destroy_super_work()
fs: Shrink only (SB_ACTIVE|SB_BORN) superblocks in super_cache_scan()
fs: Introduce struct super_operations::destroy_super() callback.
xfs: Introduce xfs_fs_destroy_super()
shmem: Implement shmem_destroy_super()
fs: Use unregister_shrinker_delayed_{initiate,finalize} for super_block shrinker

drivers/base/core.c | 42 ----------
fs/super.c | 32 ++++----
fs/xfs/xfs_super.c | 14 +++
include/linux/device.h | 2
include/linux/fs.h | 6 +
include/linux/rcutiny.h | 4 -
include/linux/shrinker.h | 2
include/linux/srcu.h | 5 -
kernel/notifier.c | 3 -
kernel/rcu/Kconfig | 12 ---
kernel/rcu/tree.h | 5 -
kernel/rcu/update.c | 4 -
mm/shmem.c | 8 ++
mm/vmscan.c | 82 ++++++++++----------
.../selftests/rcutorture/doc/TREE_RCU-kconfig.txt | 5 -
15 files changed, 89 insertions(+), 137 deletions(-)

--
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>