[PATCH v2 0/3] 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 switches count_shadow_nodes() back to lruvec_page_state_local(),
which both classic LRU and MGLRU maintain.

Patch 2 addresses the reparenting race that motivated 7404bd37cfbe:
patch 1 makes cgroup v2 read state_local as well, so extend the
dying-memcg stat redirection (previously cgroup v1 only) to all
hierarchies.

Patch 3 recovers the performance.  Patch 2 added an unconditional
rcu_read_lock() to the stat update fast path; patch 3 moves the dying
check out of the RCU read-side critical section so the lock is only
taken on the rare dying path.

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 2 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.

Patch 2 alone shows a small overhead, most visible under MGLRU (-0.6%
on w1); patch 3 brings w1 back to the base level in both LRU
configurations.  The remaining differences are within run-to-run
noise.

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

Hui Zhu (3):
  mm: workingset: use lruvec_page_state_local() to count lru pages
  mm: memcg: redirect stats updates of dying memcgs for all hierarchies
  mm: memcg: skip the RCU lock when the memcg is not dying

 mm/memcontrol.c | 30 ++++++++++++------------------
 mm/workingset.c |  5 ++---
 2 files changed, 14 insertions(+), 21 deletions(-)

-- 
2.53.0