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Date: Sun, 21 Jan 2024 21:44:12 +0000
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Message-ID: <20240121214413.833776-1-tjmercier@google.com>
Subject: [PATCH] Revert "mm:vmscan: fix inaccurate reclaim during proactive reclaim"
From: "T.J. Mercier" <tjmercier@google.com>
To: tjmercier@google.com, Johannes Weiner <hannes@cmpxchg.org>, 
	Michal Hocko <mhocko@kernel.org>, Roman Gushchin <roman.gushchin@linux.dev>, 
	Shakeel Butt <shakeelb@google.com>, Muchun Song <muchun.song@linux.dev>, 
	Andrew Morton <akpm@linux-foundation.org>
Cc: android-mm@google.com, yuzhao@google.com, yangyifei03@kuaishou.com, 
	cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
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This reverts commit 0388536ac29104a478c79b3869541524caec28eb.

Proactive reclaim on the root cgroup is 10x slower after this patch when
MGLRU is enabled, and completion times for proactive reclaim on much
smaller non-root cgroups take ~30% longer (with or without MGLRU). With
root reclaim before the patch, I observe average reclaim rates of
~70k pages/sec before try_to_free_mem_cgroup_pages starts to fail and
the nr_retries counter starts to decrement, eventually ending the
proactive reclaim attempt. After the patch the reclaim rate is
consistently ~6.6k pages/sec due to the reduced nr_pages value causing
scan aborts as soon as SWAP_CLUSTER_MAX pages are reclaimed. The
proactive reclaim doesn't complete after several minutes because
try_to_free_mem_cgroup_pages is still capable of reclaiming pages in
tiny SWAP_CLUSTER_MAX page chunks and nr_retries is never decremented.

The docs for memory.reclaim say, "the kernel can over or under reclaim
from the target cgroup" which this patch was trying to fix. Revert it
until a less costly solution is found.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
---
 mm/memcontrol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e4c8735e7c85..cee536c97151 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -6956,8 +6956,8 @@ 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),
-					GFP_KERNEL, reclaim_options);
+						nr_to_reclaim - nr_reclaimed,
+						GFP_KERNEL, reclaim_options);
 
 		if (!reclaimed && !nr_retries--)
 			return -EAGAIN;
-- 
2.43.0.429.g432eaa2c6b-goog