mm/list_lru: simplify reparenting and initial allocation

mm/list_lru: simplify reparenting and initial allocation

Currently, there is a lot of code for detecting reparent racing using
kmemcg_id as the synchronization flag.  And an intermediate table is
required to record and compare the kmemcg_id.

We can simplify this by just checking the cgroup css status, skip if
cgroup is being offlined.  On the reparenting side, ensure no more
allocation is on going and no further allocation will occur by using the
XArray lock as barrier.

Combined with a O(n^2) top-down walk for the allocation, we get rid of the
intermediate table allocation completely.  Despite being O(n^2), it should
be actually faster because it's not practical to have a very deep cgroup
level, and in most cases the parent cgroup should have been allocated
already.

This also avoided changing kmemcg_id before reparenting, making cgroups
have a stable index for list_lru_memcg.  After this change it's possible
that a dying cgroup will see a NULL value in XArray corresponding to the
kmemcg_id, because the kmemcg_id will point to an empty slot.  In such
case, just fallback to use its parent.

As a result the code is simpler, following test also showed a very slight
performance gain (12 test runs):

prepare() {
        mkdir /tmp/test-fs
        modprobe brd rd_nr=1 rd_size=16777216
        mkfs.xfs -f /dev/ram0
        mount -t xfs /dev/ram0 /tmp/test-fs
        for i in $(seq 10000); do
                seq 8000 > "/tmp/test-fs/$i"
        done
        mkdir -p /sys/fs/cgroup/system.slice/bench/test/1
        echo +memory > /sys/fs/cgroup/system.slice/bench/cgroup.subtree_control
        echo +memory > /sys/fs/cgroup/system.slice/bench/test/cgroup.subtree_control
        echo +memory > /sys/fs/cgroup/system.slice/bench/test/1/cgroup.subtree_control
        echo 768M > /sys/fs/cgroup/system.slice/bench/memory.max
}

do_test() {
        read_worker() {
                mkdir -p "/sys/fs/cgroup/system.slice/bench/test/1/$1"
                echo $BASHPID > "/sys/fs/cgroup/system.slice/bench/test/1/$1/cgroup.procs"
                read -r __TMP < "/tmp/test-fs/$1";
        }
        read_in_all() {
                for i in $(seq 10000); do
                        read_worker "$i" &
                done; wait
        }
        echo 3 > /proc/sys/vm/drop_caches
        time read_in_all
        for i in $(seq 1 10000); do
                rmdir "/sys/fs/cgroup/system.slice/bench/test/1/$i" &>/dev/null
        done
}

Before:
real    0m3.498s   user    0m11.037s  sys     0m35.872s
real    1m33.860s  user    0m11.593s  sys     3m1.169s
real    1m31.883s  user    0m11.265s  sys     2m59.198s
real    1m32.394s  user    0m11.294s  sys     3m1.616s
real    1m31.017s  user    0m11.379s  sys     3m1.349s
real    1m31.931s  user    0m11.295s  sys     2m59.863s
real    1m32.758s  user    0m11.254s  sys     2m59.538s
real    1m35.198s  user    0m11.145s  sys     3m1.123s
real    1m30.531s  user    0m11.393s  sys     2m58.089s
real    1m31.142s  user    0m11.333s  sys     3m0.549s

After:
real    0m3.489s   user    0m10.943s  sys     0m36.036s
real    1m10.893s  user    0m11.495s  sys     2m38.545s
real    1m29.129s  user    0m11.382s  sys     3m1.601s
real    1m29.944s  user    0m11.494s  sys     3m1.575s
real    1m31.208s  user    0m11.451s  sys     2m59.693s
real    1m25.944s  user    0m11.327s  sys     2m56.394s
real    1m28.599s  user    0m11.312s  sys     3m0.162s
real    1m26.746s  user    0m11.538s  sys     2m55.462s
real    1m30.668s  user    0m11.475s  sys     3m2.075s
real    1m29.258s  user    0m11.292s  sys     3m0.780s

Which is slightly faster in real time.

Link: https://lkml.kernel.org/r/20241104175257.60853-5-ryncsn@gmail.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Waiman Long <longman@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>