linux-kernel - [PATCH] mm: workingset: ignore slab memory size when calculating shadows pressure

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Message-ID: <20200903230055.1245058-1-guro@fb.com>
Date:   Thu, 3 Sep 2020 16:00:55 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>
CC:     =Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>, Roman Gushchin <guro@...com>,
        Michal Hocko <mhocko@...e.com>
Subject: [PATCH] mm: workingset: ignore slab memory size when calculating shadows pressure

In the memcg case count_shadow_nodes() sums the number of pages in lru
lists and the amount of slab memory (reclaimable and non-reclaimable)
as a baseline for the allowed number of shadow entries.

It seems to be a good analogy for the !memcg case, where
node_present_pages() is used. However, it's not quite true, as there
two problems:

1) Due to slab reparenting introduced by commit fb2f2b0adb98 ("mm:
memcg/slab: reparent memcg kmem_caches on cgroup removal") local
per-lruvec slab counters might be inaccurate on non-leaf levels.
It's the only place where local slab counters are used.

2) Shadow nodes by themselves are backed by slabs. So there is a loop
dependency: the more shadow entries are there, the less pressure the
kernel applies to reclaim them.

Fortunately, there is a simple way to solve both problems: slab
counters shouldn't be taken into the account by count_shadow_nodes().

Signed-off-by: Roman Gushchin <guro@...com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Shakeel Butt <shakeelb@...gle.com>
---
 mm/workingset.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/workingset.c b/mm/workingset.c
index 92e66113a577..50d53f3699e4 100644
--- a/mm/workingset.c
+++ b/mm/workingset.c
@@ -495,10 +495,6 @@ static unsigned long count_shadow_nodes(struct shrinker *shrinker,
 		for (pages = 0, i = 0; i < NR_LRU_LISTS; i++)
 			pages += lruvec_page_state_local(lruvec,
 							 NR_LRU_BASE + i);
-		pages += lruvec_page_state_local(
-			lruvec, NR_SLAB_RECLAIMABLE_B) >> PAGE_SHIFT;
-		pages += lruvec_page_state_local(
-			lruvec, NR_SLAB_UNRECLAIMABLE_B) >> PAGE_SHIFT;
 	} else
 #endif
 		pages = node_present_pages(sc->nid);
-- 
2.26.2