[PATCH v4 0/4] mm: workingset: fix the shadow node budget under MGLRU

[Hui Zhu <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

Commit 7404bd37cfbe ("mm: workingset: use lruvec_lru_size() to get the
number of lru pages") broke the workingset shadow node budget under
MGLRU: lruvec_lru_size() reads mz->lru_zone_size, which MGLRU never
maintains, so count_shadow_nodes() sees the evictable LRU lists as
empty and the shadow shrinker reclaims eviction tokens almost as fast
as they are created, losing thrashing protection.

Patch 1 extends the dying-mcg stat redirection (previously cgroup v1
only) to all hierarchies, addressing the reparenting race that motivated
7404bd37cfbe.

Patch 2 then switches count_shadow_nodes() back to
lruvec_page_state_local(), which both classic LRU and MGLRU maintain.

Patch 3 recovers the performance.  Patch 1 added an unconditional
rcu_read_lock() to the stat update fast path; patch 3 checks
memcg_is_dying() first and takes the RCU lock only on the rare dying
path.

Patch 4 closes an accounting gap that patch 2 makes visible: on cgroup
v2, reparent_state_local() never moves the dying memcg's
non-hierarchical lruvec stats to the parent, so the parent receives
the uncharges without the matching charges and its state_local
underflows.  Patch 4 reparents those stats, mirroring cgroup v1.

Performance testing
===================

The test script and the raw results are available at [1].

Environment: 10-vCPU QEMU guest, 8 GiB RAM, cgroup v2; 7 runs per
configuration, medians reported.  Workloads:

  w1-anon-churn: single-threaded anon fault/charge loop in a memcg
                 (MADV_DONTNEED + re-fault, no reclaim).  Every touch
                 is a real fault with charge and memcg stat updates,
                 so it stresses exactly the fast path patch 1 changes.
  w2-file-churn: file read loop under memory.high pressure
                 (reclaim-bound, noisier).
  w3-reparent:   reparent accounting sanity check.

w1-anon-churn (pages/s):

                 classic LRU          MGLRU
base             4393028              4377122
patches 1-2      4385996   (-0.2%)    4352887   (-0.6%)
patches 1-3      4381832   (-0.3%)    4377053   (+0.0%)

w2-file-churn (MB/s):

                 classic LRU          MGLRU
base             8277                 8226
patches 1-2      8226      (-0.6%)    8123      (-1.3%)
patches 1-3      8157      (-1.4%)    8294      (+0.8%)

w3-reparent passed on all kernels.

The small overhead visible with patches 1-2 comes from the redirection
added by patch 1; patch 3 brings w1 back to the base level in both LRU
configurations.  The remaining differences are within run-to-run noise.
Patch 4 only touches the memcg offline path and is not exercised by
these workloads.

[1] https://gist.github.com/teawater/32f373ec41d185d840455eb167321a5a

Changelog:
v4:
According to the comments of Andrew, Fix "follow-up patch" to
"preceding patch" in the commit message to match the reordered series.
According to the comments of Sashiko, add patch 4 to reparent the
non-hierarchical lruvec stats on cgroup v2 to fixing the state_local
underflow.
v3:
According to the comments of Shakeel, reorder the series per review,
add Fixes/Cc stable and the user-visible impact to patch 1.

Hui Zhu (4):
  mm: memcg: redirect stats updates of dying memcgs for all hierarchies
  mm: workingset: use lruvec_page_state_local() to count lru pages
  mm: memcg: skip the RCU lock when the memcg is not dying
  mm: memcg: reparent non-hierarchical lruvec stats on cgroup v2

 mm/memcontrol-v1.h |  5 ++--
 mm/memcontrol.c    | 72 +++++++++++++++++++++++++---------------------
 mm/workingset.c    |  5 ++--
 3 files changed, 45 insertions(+), 37 deletions(-)

-- 
2.43.0