[PATCH v2] mm: memcg: Use larger batches for proactive reclaim

["T.J. Mercier" <tjmercier@google.com>]

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 0388536ac291 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.

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.

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@google.com>

---
v2: Simplify the request size calculation per Johannes Weiner and Michal Koutný

 mm/memcontrol.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 46d8d02114cf..e6f921555e07 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6965,6 +6965,9 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
 	while (nr_reclaimed < nr_to_reclaim) {
 		unsigned long reclaimed;
 
+		/* Will converge on zero, but reclaim enforces a minimum */
+		unsigned long batch_size = (nr_to_reclaim - nr_reclaimed) / 4;
+
 		if (signal_pending(current))
 			return -EINTR;
 
@@ -6977,7 +6980,7 @@ 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),
+					batch_size,
 					GFP_KERNEL, reclaim_options);
 
 		if (!reclaimed && !nr_retries--)
-- 
2.43.0.594.gd9cf4e227d-goog