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From:   Kent Overstreet <kent.overstreet@gmail.com>
To:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org
Cc:     Kent Overstreet <kent.overstreet@gmail.com>,
        roman.gushchin@linux.dev, hannes@cmpxchg.org
Subject: [PATCH 2/4] mm: Add a .to_text() method for shrinkers
Date:   Thu, 21 Apr 2022 19:48:26 -0400
Message-Id: <20220421234837.3629927-3-kent.overstreet@gmail.com>
In-Reply-To: <20220421234837.3629927-1-kent.overstreet@gmail.com>
References: <20220421234837.3629927-1-kent.overstreet@gmail.com>
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This adds a new callback method to shrinkers which they can use to
describe anything relevant to memory reclaim about their internal state,
for example object dirtyness.

This uses the new printbufs to output to heap allocated strings, so that
the .to_text() methods can be used both for messages logged to the
console, and also sysfs/debugfs.

This patch also adds shrinkers_to_text(), which reports on the top 10
shrinkers - by object count - in sorted order, to be used in OOM
reporting.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
---
 include/linux/shrinker.h |  5 +++
 mm/vmscan.c              | 75 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 76fbf92b04..b5f411768b 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -2,6 +2,8 @@
 #ifndef _LINUX_SHRINKER_H
 #define _LINUX_SHRINKER_H
 
+struct printbuf;
+
 /*
  * This struct is used to pass information from page reclaim to the shrinkers.
  * We consolidate the values for easier extension later.
@@ -58,10 +60,12 @@ struct shrink_control {
  * @flags determine the shrinker abilities, like numa awareness
  */
 struct shrinker {
+	char name[32];
 	unsigned long (*count_objects)(struct shrinker *,
 				       struct shrink_control *sc);
 	unsigned long (*scan_objects)(struct shrinker *,
 				      struct shrink_control *sc);
+	void (*to_text)(struct printbuf *, struct shrinker *);
 
 	long batch;	/* reclaim batch size, 0 = default */
 	int seeks;	/* seeks to recreate an obj */
@@ -94,4 +98,5 @@ extern int register_shrinker(struct shrinker *shrinker);
 extern void unregister_shrinker(struct shrinker *shrinker);
 extern void free_prealloced_shrinker(struct shrinker *shrinker);
 extern void synchronize_shrinkers(void);
+void shrinkers_to_text(struct printbuf *);
 #endif
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 59b14e0d69..09c483dfd3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -50,6 +50,7 @@
 #include <linux/printk.h>
 #include <linux/dax.h>
 #include <linux/psi.h>
+#include <linux/printbuf.h>
 
 #include <asm/tlbflush.h>
 #include <asm/div64.h>
@@ -702,6 +703,80 @@ void synchronize_shrinkers(void)
 }
 EXPORT_SYMBOL(synchronize_shrinkers);
 
+/**
+ * shrinkers_to_text - Report on shrinkers with highest usage
+ *
+ * This reports on the top 10 shrinkers, by object counts, in sorted order:
+ * intended to be used for OOM reporting.
+ */
+void shrinkers_to_text(struct printbuf *out)
+{
+	struct shrinker *shrinker;
+	struct shrinker_by_mem {
+		struct shrinker	*shrinker;
+		unsigned long	mem;
+	} shrinkers_by_mem[10];
+	int i, nr = 0;
+
+	if (!down_read_trylock(&shrinker_rwsem)) {
+		pr_buf(out, "(couldn't take shrinker lock)");
+		return;
+	}
+
+	list_for_each_entry(shrinker, &shrinker_list, list) {
+		struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
+		unsigned long mem = shrinker->count_objects(shrinker, &sc);
+
+		if (!mem || mem == SHRINK_STOP || mem == SHRINK_EMPTY)
+			continue;
+
+		for (i = 0; i < nr; i++)
+			if (mem < shrinkers_by_mem[i].mem)
+				break;
+
+		if (nr < ARRAY_SIZE(shrinkers_by_mem)) {
+			memmove(&shrinkers_by_mem[i + 1],
+				&shrinkers_by_mem[i],
+				sizeof(shrinkers_by_mem[0]) * (nr - i));
+			nr++;
+		} else if (i) {
+			i--;
+			memmove(&shrinkers_by_mem[0],
+				&shrinkers_by_mem[1],
+				sizeof(shrinkers_by_mem[0]) * i);
+		} else {
+			continue;
+		}
+
+		shrinkers_by_mem[i] = (struct shrinker_by_mem) {
+			.shrinker = shrinker,
+			.mem = mem,
+		};
+	}
+
+	for (i = nr - 1; i >= 0; --i) {
+		struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
+		shrinker = shrinkers_by_mem[i].shrinker;
+
+		if (shrinker->name[0])
+			pr_buf(out, "%s", shrinker->name);
+		else
+			pr_buf(out, "%ps:", shrinker->scan_objects);
+
+		pr_buf(out, " objects: %lu", shrinker->count_objects(shrinker, &sc));
+		pr_newline(out);
+
+		if (shrinker->to_text) {
+			pr_indent_push(out, 2);
+			shrinker->to_text(out, shrinker);
+			pr_indent_pop(out, 2);
+			pr_newline(out);
+		}
+	}
+
+	up_read(&shrinker_rwsem);
+}
+
 #define SHRINK_BATCH 128
 
 static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
-- 
2.35.2