[PATCH v4] mm: memcg: Use larger batches for proactive reclaim

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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 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. 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-0388536ac291      :    68047        10.46
post-0388536ac291     :    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-0388536ac291      :    258822       1.12            107.8
post-0388536ac291     :    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-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>
Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>

v4: Add additional info to commit message and move definition of
batch_size per Michal Hocko. No functional changes.
v3: Formatting fixes per Yosry Ahmed and Johannes Weiner. No functional
v2: Simplify the request size calculation per Johannes Weiner and Michal Koutný

 mm/memcontrol.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 46d8d02114cf..02b054a316d3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6963,6 +6963,8 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
 	while (nr_reclaimed < nr_to_reclaim) {
+		/* Will converge on zero, but reclaim enforces a minimum */
+		unsigned long batch_size = (nr_to_reclaim - nr_reclaimed) / 4;
 		unsigned long reclaimed;
 		if (signal_pending(current))
@@ -6977,8 +6979,7 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
 		reclaimed = try_to_free_mem_cgroup_pages(memcg,
-					min(nr_to_reclaim - nr_reclaimed, SWAP_CLUSTER_MAX),
-					GFP_KERNEL, reclaim_options);
+					batch_size, GFP_KERNEL, reclaim_options);
 		if (!reclaimed && !nr_retries--)
 			return -EAGAIN;

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