Re: [PATCH 02/10] mm: shrinker: Add a .to_text() method for shrinkers

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On 2024/8/26 11:04, Kent Overstreet wrote:
On Mon, Aug 26, 2024 at 11:01:50AM GMT, Qi Zheng wrote:


On 2024/8/25 03:10, Kent Overstreet wrote:
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 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.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
---
   include/linux/shrinker.h |  7 +++-
   mm/shrinker.c            | 73 +++++++++++++++++++++++++++++++++++++++-
   2 files changed, 78 insertions(+), 2 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 1a00be90d93a..6193612617a1 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -24,6 +24,8 @@ struct shrinker_info {
   	struct shrinker_info_unit *unit[];
   };
+struct seq_buf;
+
   /*
    * This struct is used to pass information from page reclaim to the shrinkers.
    * We consolidate the values for easier extension later.
@@ -80,10 +82,12 @@ struct shrink_control {
    * @flags determine the shrinker abilities, like numa awareness
    */
   struct shrinker {
+	const char *name;
   	unsigned long (*count_objects)(struct shrinker *,
   				       struct shrink_control *sc);
   	unsigned long (*scan_objects)(struct shrinker *,
   				      struct shrink_control *sc);
+	void (*to_text)(struct seq_buf *, struct shrinker *);
   	long batch;	/* reclaim batch size, 0 = default */
   	int seeks;	/* seeks to recreate an obj */
@@ -110,7 +114,6 @@ struct shrinker {
   #endif
   #ifdef CONFIG_SHRINKER_DEBUG
   	int debugfs_id;
-	const char *name;
   	struct dentry *debugfs_entry;
   #endif
   	/* objs pending delete, per node */
@@ -135,6 +138,8 @@ __printf(2, 3)
   struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...);
   void shrinker_register(struct shrinker *shrinker);
   void shrinker_free(struct shrinker *shrinker);
+void shrinker_to_text(struct seq_buf *, struct shrinker *);
+void shrinkers_to_text(struct seq_buf *);
   static inline bool shrinker_try_get(struct shrinker *shrinker)
   {
diff --git a/mm/shrinker.c b/mm/shrinker.c
index dc5d2a6fcfc4..ad52c269bb48 100644
--- a/mm/shrinker.c
+++ b/mm/shrinker.c
@@ -1,8 +1,9 @@
   // SPDX-License-Identifier: GPL-2.0
   #include <linux/memcontrol.h>
+#include <linux/rculist.h>
   #include <linux/rwsem.h>
+#include <linux/seq_buf.h>
   #include <linux/shrinker.h>
-#include <linux/rculist.h>
   #include <trace/events/vmscan.h>
   #include "internal.h"
@@ -807,3 +808,73 @@ void shrinker_free(struct shrinker *shrinker)
   	call_rcu(&shrinker->rcu, shrinker_free_rcu_cb);
   }
   EXPORT_SYMBOL_GPL(shrinker_free);
+
+void shrinker_to_text(struct seq_buf *out, struct shrinker *shrinker)
+{
+	struct shrink_control sc = { .gfp_mask = GFP_KERNEL, };
+
+	seq_buf_puts(out, shrinker->name);
+	seq_buf_printf(out, " objects: %lu\n", shrinker->count_objects(shrinker, &sc));
+
+	if (shrinker->to_text) {
+		shrinker->to_text(out, shrinker);
+		seq_buf_puts(out, "\n");
+	}
+}
+
+/**
+ * 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 seq_buf *out)
+{
+	struct shrinker *shrinker;
+	struct shrinker_by_mem {
+		struct shrinker	*shrinker;
+		unsigned long	mem;
+	} shrinkers_by_mem[10];
+	int i, nr = 0;
+
+	if (!mutex_trylock(&shrinker_mutex)) {
+		seq_buf_puts(out, "(couldn't take shrinker lock)");
+		return;
+	}

I remember I pointed out that the RCU + refcount method should be used
here. Otherwise you will block other shrinkers from
registering/unregistering, etc.

The more complex iteration isn't needed here - this is a slowpath
function and we're not doing anything blocking inside it.

The shrinker list may be very long, and can you guarantee that shrinkers
other than bcachefs won't do anything blocking? ;)




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