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Before 388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive
reclaim") we passed the number of pages for the reclaim request directly
to try_to_free_mem_cgroup_pages, which could lead to significant
overreclaim.  After 0388536a the number of pages was limited to a
maximum 32 (SWAP_CLUSTER_MAX) to reduce the amount of overreclaim. 
However such a small batch size caused a regression in reclaim performance
due to many more reclaim start/stop cycles inside memory_reclaim.  The
restart cost is amortized over more pages with larger batch sizes, and
becomes a significant component of the runtime if the batch size is too
small.

Reclaim tries to balance nr_to_reclaim fidelity with fairness across nodes
and cgroups over which the pages are spread.  As such, the bigger the
request, the bigger the absolute overreclaim error.  Historic in-kernel
users of reclaim have used fixed, small sized requests to approach an
appropriate reclaim rate over time.  When we reclaim a user request of
arbitrary size, use decaying batch sizes to manage error while maintaining
reasonable throughput.

MGLRU enabled - memcg LRU used
root - full reclaim       pages/sec   time (sec)
pre-0388536a      :    68047        10.46
post-0388536a     :    13742        inf
(reclaim-reclaimed)/4 :    67352        10.51

MGLRU enabled - memcg LRU not used
/uid_0 - 1G reclaim       pages/sec   time (sec)  overreclaim (MiB)
pre-0388536a      :    258822       1.12            107.8
post-0388536a     :    105174       2.49            3.5
(reclaim-reclaimed)/4 :    233396       1.12            -7.4

MGLRU enabled - memcg LRU not used
/uid_0 - full reclaim     pages/sec   time (sec)
pre-0388536a      :    72334        7.09
post-0388536a     :    38105        14.45
(reclaim-reclaimed)/4 :    72914        6.96

[tjmercier@google.com: v4]
  Link: https://lkml.kernel.org/r/20240206175251.3364296-1-tjmercier@google.com
Link: https://lkml.kernel.org/r/20240202233855.1236422-1-tjmercier@google.com
Fixes: 0388536a

 ("mm:vmscan: fix inaccurate reclaim during proactive reclaim")
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Michal Koutny <mkoutny@suse.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Efly Young <yangyifei03@kuaishou.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>