+ mm-list_lru-optimize-memcg_reparent_list_lru_node.patch added to -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/list_lru: optimize memcg_reparent_list_lru_node()
has been added to the -mm tree.  Its filename is
     mm-list_lru-optimize-memcg_reparent_list_lru_node.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-list_lru-optimize-memcg_reparent_list_lru_node.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-list_lru-optimize-memcg_reparent_list_lru_node.patch

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------------------------------------------------------
From: Waiman Long <longman@redhat.com>
Subject: mm/list_lru: optimize memcg_reparent_list_lru_node()

Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() to be
race free"), we are tracking the total number of lru entries in a
list_lru_node in its nr_items field.  In the case of
memcg_reparent_list_lru_node(), there is nothing to be done if nr_items is
0.  We don't even need to take the nlru->lock as no new lru entry could be
added by a racing list_lru_add() to the draining src_idx memcg at this
point.

On systems that serve a lot of containers, it is possible that there can
be thousands of list_lru's present due to the fact that each container may
mount its own container specific filesystems.  As a typical container uses
only a few cpus, it is likely that only the list_lru_node that contains
those cpus will be utilized while the rests may be empty.  In other words,
there can be a lot of list_lru_node with 0 nr_items.  By skipping a
lock/unlock operation and loading a cacheline from memcg_lrus, a sizeable
number of cpu cycles can be saved.  That can be substantial if we are
talking about thousands of list_lru_node's with 0 nr_items.

Link: https://lkml.kernel.org/r/20220309144000.1470138-1-longman@redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/list_lru.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/mm/list_lru.c~mm-list_lru-optimize-memcg_reparent_list_lru_node
+++ a/mm/list_lru.c
@@ -519,6 +519,12 @@ static void memcg_drain_list_lru_node(st
 	struct list_lru_one *src, *dst;
 
 	/*
+	 * If there is no lru entry in this nlru, we can skip it immediately.
+	 */
+	if (!READ_ONCE(nlru->nr_items))
+		return;
+
+	/*
 	 * Since list_lru_{add,del} may be called under an IRQ-safe lock,
 	 * we have to use IRQ-safe primitives here to avoid deadlock.
 	 */
_

Patches currently in -mm which might be from longman@redhat.com are

mm-list_lru-optimize-memcg_reparent_list_lru_node.patch
lib-vsprintf-avoid-redundant-work-with-0-size.patch
mm-page_owner-use-scnprintf-to-avoid-excessive-buffer-overrun-check.patch
mm-page_owner-print-memcg-information.patch
mm-page_owner-record-task-command-name.patch
ipc-mqueue-use-get_tree_nodev-in-mqueue_get_tree.patch