From 287d5fedb377ddc232b216b882723305b27ae31a Mon Sep 17 00:00:00 2001 From: "T.J. Mercier" Date: Fri, 2 Feb 2024 23:38:54 +0000 Subject: [PATCH] mm: memcg: use larger batches for proactive reclaim 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 [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: 0388536ac291 ("mm:vmscan: fix inaccurate reclaim during proactive reclaim") Signed-off-by: T.J. Mercier Reviewed-by: Yosry Ahmed Acked-by: Johannes Weiner Reviewed-by: Michal Koutny Acked-by: Shakeel Butt Acked-by: Michal Hocko Cc: Roman Gushchin Cc: Shakeel Butt Cc: Muchun Song Cc: Efly Young Cc: Yu Zhao Signed-off-by: Andrew Morton --- mm/memcontrol.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 484a9d2862d4f..cb216d30a2215 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -6981,6 +6981,8 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf, reclaim_options = MEMCG_RECLAIM_MAY_SWAP | MEMCG_RECLAIM_PROACTIVE; 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)) @@ -6995,8 +6997,7 @@ 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), - GFP_KERNEL, reclaim_options); + batch_size, GFP_KERNEL, reclaim_options); if (!reclaimed && !nr_retries--) return -EAGAIN; -- 2.30.2