mm: vmscan: make memcg slab shrink lockless

Like global slab shrink, this commit also uses SRCU to make memcg slab
shrink lockless.

We can reproduce the down_read_trylock() hotspot through the
following script:

```

DIR="/root/shrinker/memcg/mnt"

do_create()
{
    mkdir -p /sys/fs/cgroup/memory/test
    mkdir -p /sys/fs/cgroup/perf_event/test
    echo 4G > /sys/fs/cgroup/memory/test/memory.limit_in_bytes
    for i in `seq 0 $1`;
    do
        mkdir -p /sys/fs/cgroup/memory/test/$i;
        echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
        echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
        mkdir -p $DIR/$i;
    done
}

do_mount()
{
    for i in `seq $1 $2`;
    do
        mount -t tmpfs $i $DIR/$i;
    done
}

do_touch()
{
    for i in `seq $1 $2`;
    do
        echo $$ > /sys/fs/cgroup/memory/test/$i/cgroup.procs;
        echo $$ > /sys/fs/cgroup/perf_event/test/cgroup.procs;
            dd if=/dev/zero of=$DIR/$i/file$i bs=1M count=1 &
    done
}

case "$1" in
  touch)
    do_touch $2 $3
    ;;
  test)
      do_create 4000
    do_mount 0 4000
    do_touch 0 3000
    ;;
  *)
    exit 1
    ;;
esac
```

Save the above script, then run test and touch commands.
Then we can use the following perf command to view hotspots:

perf top -U -F 999

1) Before applying this patchset:

  32.31%  [kernel]           [k] down_read_trylock
  19.40%  [kernel]           [k] pv_native_safe_halt
  16.24%  [kernel]           [k] up_read
  15.70%  [kernel]           [k] shrink_slab
   4.69%  [kernel]           [k] _find_next_bit
   2.62%  [kernel]           [k] shrink_node
   1.78%  [kernel]           [k] shrink_lruvec
   0.76%  [kernel]           [k] do_shrink_slab

2) After applying this patchset:

  27.83%  [kernel]           [k] _find_next_bit
  16.97%  [kernel]           [k] shrink_slab
  15.82%  [kernel]           [k] pv_native_safe_halt
   9.58%  [kernel]           [k] shrink_node
   8.31%  [kernel]           [k] shrink_lruvec
   5.64%  [kernel]           [k] do_shrink_slab
   3.88%  [kernel]           [k] mem_cgroup_iter

At the same time, we use the following perf command to capture
IPC information:

perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10

1) Before applying this patchset:

 Performance counter stats for 'system wide' (5 runs):

      454187219766      cycles                    test                    ( +-  1.84% )
       78896433101      instructions              test #    0.17  insn per cycle           ( +-  0.44% )

        10.0020430 +- 0.0000366 seconds time elapsed  ( +-  0.00% )

2) After applying this patchset:

 Performance counter stats for 'system wide' (5 runs):

      841954709443      cycles                    test                    ( +- 15.80% )  (98.69%)
      527258677936      instructions              test #    0.63  insn per cycle           ( +- 15.11% )  (98.68%)

          10.01064 +- 0.00831 seconds time elapsed  ( +-  0.08% )

We can see that IPC drops very seriously when calling
down_read_trylock() at high frequency. After using SRCU,
the IPC is at a normal level.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Qi Zheng <[email protected]>
Acked-by: Kirill Tkhai <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Cc: Christian König <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Sultan Alsawaf <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Cc: Yang Shi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

0 files changed, 0 insertions, 0 deletions