[PATCH mm-unstable v2 0/8] mm: multi-gen LRU: memcg LRU

[Yu Zhao]

What's new
==========
1. Rebased to the latest mm-unstable.
2. Added two comprehensive benchmarks:
   https://lore.kernel.org/r/20221220214923.1229538-1-yuzhao@xxxxxxxxxx/
   https://lore.kernel.org/r/20221221000748.1374772-1-yuzhao@xxxxxxxxxx/

Overview
========
An memcg LRU is a per-node LRU of memcgs. It is also an LRU of LRUs,
since each node and memcg combination has an LRU of folios (see
mem_cgroup_lruvec()).

Its goal is to improve the scalability of global reclaim, which is
critical to system-wide memory overcommit in data centers. Note that
memcg reclaim is currently out of scope.

Its memory bloat is a pointer to each lruvec and negligible to each
pglist_data. In terms of traversing memcgs during global reclaim, it
improves the best-case complexity from O(n) to O(1) and does not
affect the worst-case complexity O(n). Therefore, on average, it has
a sublinear complexity in contrast to the current linear complexity.

The basic structure of an memcg LRU can be understood by an analogy
to the active/inactive LRU (of folios):
1. It has the young and the old (generations), the counterparts to
   the active and the inactive;
2. The increment of max_seq triggers promotion, the counterpart to
   activation;
3. Other events, e.g., offlining an memcg, triggers similar
   operations, e.g., demotion, the counterpart to deactivation.

In terms of global reclaim, it has two distinct features:
1. Sharding, which allows each thread to start at a random memcg (in
   the old generation) and improves parallelism;
2. Eventual fairness, which allows direct reclaim to bail out and
   reduces latency without affecting fairness over some time.

The commit message in patch 6 details the workflow:
https://lore.kernel.org/r/20221221001207.1376119-7-yuzhao@xxxxxxxxxx/

The following is a simple test to quickly verify its effectiveness.
More benchmarks are coming soon.

  Test design:
  1. Create multiple memcgs.
  2. Each memcg contains a job (fio).
  3. All jobs access the same amount of memory randomly.
  4. The system does not experience global memory pressure.
  5. Periodically write to the root memory.reclaim.

  Desired outcome:
  1. All memcgs have similar pgsteal counts, i.e.,
     stddev(pgsteal)/mean(pgsteal) is close to 0%.
  2. The total pgsteal is close to the total requested through
     memory.reclaim, i.e., sum(pgsteal)/sum(requested) is close to
     100%.

  Actual outcome [1]:
                                     MGLRU off    MGLRU on
  stddev(pgsteal) / mean(pgsteal)    75%          20%
  sum(pgsteal) / sum(requested)      425%         95%
  ####################################################################
  MEMCGS=128

  for ((memcg = 0; memcg < $MEMCGS; memcg++)); do
      mkdir /sys/fs/cgroup/memcg$memcg
  done

  start() {
      echo $BASHPID > /sys/fs/cgroup/memcg$memcg/cgroup.procs

      fio -name=memcg$memcg --numjobs=1 --ioengine=mmap \
          --filename=/dev/zero --size=1920M --rw=randrw \
          --rate=64m,64m --random_distribution=random \
          --fadvise_hint=0 --time_based --runtime=10h \
          --group_reporting --minimal
  }

  for ((memcg = 0; memcg < $MEMCGS; memcg++)); do
      start &
  done

  sleep 600

  for ((i = 0; i < 600; i++)); do
      echo 256m >/sys/fs/cgroup/memory.reclaim
      sleep 6
  done

  for ((memcg = 0; memcg < $MEMCGS; memcg++)); do
      grep "pgsteal " /sys/fs/cgroup/memcg$memcg/memory.stat
  done
  ####################################################################

[1]: This was obtained from running the above script (touches less
     than 256GB memory) on an EPYC 7B13 with 512GB DRAM for over an
     hour.

Yu Zhao (8):
  mm: multi-gen LRU: rename lru_gen_struct to lru_gen_folio
  mm: multi-gen LRU: rename lrugen->lists[] to lrugen->folios[]
  mm: multi-gen LRU: remove eviction fairness safeguard
  mm: multi-gen LRU: remove aging fairness safeguard
  mm: multi-gen LRU: shuffle should_run_aging()
  mm: multi-gen LRU: per-node lru_gen_folio lists
  mm: multi-gen LRU: clarify scan_control flags
  mm: multi-gen LRU: simplify arch_has_hw_pte_young() check

 Documentation/mm/multigen_lru.rst |   8 +-
 include/linux/memcontrol.h        |  10 +
 include/linux/mm_inline.h         |  25 +-
 include/linux/mmzone.h            | 131 ++++-
 mm/memcontrol.c                   |  16 +
 mm/page_alloc.c                   |   1 +
 mm/vmscan.c                       | 768 ++++++++++++++++++++----------
 mm/workingset.c                   |   4 +-
 8 files changed, 692 insertions(+), 271 deletions(-)

-- 
2.39.0.314.g84b9a713c41-goog