[PATCH] mm: memcg: Use larger chunks for proactive reclaim

[T.J. Mercier]

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 in order to achieve fairness. After 0388536ac291 the number
of pages was limited to a maxmimum of 32 (SWAP_CLUSTER_MAX) to reduce
the amount of overreclaim. However such a small chunk size caused a
regression in reclaim performance due to many more reclaim start/stop
cycles inside memory_reclaim.

Instead of limiting reclaim chunk size to the SWAP_CLUSTER_MAX constant,
adjust the chunk size proportionally with number of pages left to
reclaim. This allows for higher reclaim efficiency with large chunk
sizes during the beginning of memory_reclaim, and reduces the amount of
potential overreclaim by using small chunk sizes as the total reclaim
amount is approached. Using 1/4 of the amount left to reclaim as the
chunk size gives a good compromise between reclaim performance and
overreclaim:

root - full reclaim       pages/sec   time (sec)
pre-0388536ac291      :    68047        10.46
post-0388536ac291     :    13742        inf
(reclaim-reclaimed)/4 :    67352        10.51

/uid_0 - 1G reclaim       pages/sec   time (sec)  overreclaim (MiB)
pre-0388536ac291      :    258822       1.12            107.8
post-0388536ac291     :    105174       2.49            3.5
(reclaim-reclaimed)/4 :    233396       1.12            -7.4

/uid_0 - full reclaim     pages/sec   time (sec)
pre-0388536ac291      :    72334        7.09
post-0388536ac291     :    38105        14.45
(reclaim-reclaimed)/4 :    72914        6.96

Fixes: 0388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive reclaim")
Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
---
 mm/memcontrol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 46d8d02114cf..d68fb89eadd2 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6977,7 +6977,8 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
 			lru_add_drain_all();
 
 		reclaimed = try_to_free_mem_cgroup_pages(memcg,
-					min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
+					max((nr_to_reclaim - nr_reclaimed) / 4,
+					    (nr_to_reclaim - nr_reclaimed) % 4),
 					GFP_KERNEL, reclaim_options);
 
 		if (!reclaimed && !nr_retries--)
-- 
2.43.0.594.gd9cf4e227d-goog