From 973086759974f36987e3c0e9c78bb1e9a41661cc Mon Sep 17 00:00:00 2001
From: Scott Cheloha <cheloha@linux.vnet.ibm.com>
Date: Tue, 17 Dec 2019 13:32:38 -0600
Subject: [PATCH] drivers/base/memory.c: cache blocks in radix tree to
 accelerate lookup

References: bsc#1159955 ltc#182993
Patch-mainline: submitted https://lore.kernel.org/lkml/20191217193238.3098-1-cheloha@linux.vnet.ibm.com/

Searching for a particular memory block by id is slow because each block
device is kept in an unsorted linked list on the subsystem bus.

Lookup is much faster if we cache the blocks in a radix tree.  Memory
subsystem initialization and hotplug/hotunplug is at least a little faster
for any machine with more than ~100 blocks, and the speedup grows with
the block count.

Signed-off-by: Scott Cheloha <cheloha@linux.vnet.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Michal Suchanek <msuchanek@suse.de>
---
 drivers/base/memory.c | 36 +++++++++++++++++++++++-------------
 1 file changed, 23 insertions(+), 13 deletions(-)

diff --git a/drivers/base/memory.c b/drivers/base/memory.c
index b2183cdab117..ae6dc5bcd125 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -20,6 +20,7 @@
 #include <linux/memory_hotplug.h>
 #include <linux/mm.h>
 #include <linux/mutex.h>
+#include <linux/radix-tree.h>
 #include <linux/stat.h>
 #include <linux/slab.h>
 
@@ -59,6 +60,13 @@ static struct bus_type memory_subsys = {
 	.offline = memory_subsys_offline,
 };
 
+/*
+ * Memory blocks are cached in a local radix tree to avoid
+ * a costly linear search for the corresponding device on
+ * the subsystem bus.
+ */
+static RADIX_TREE(memory_blocks, GFP_KERNEL);
+
 static BLOCKING_NOTIFIER_HEAD(memory_chain);
 
 int register_memory_notifier(struct notifier_block *nb)
@@ -589,20 +597,14 @@ int __weak arch_get_memory_phys_device(unsigned long start_pfn)
 /* A reference for the returned memory block device is acquired. */
 static struct memory_block *find_memory_block_by_id(unsigned long block_id)
 {
-	struct device *dev;
+	struct memory_block *mem;
 
-	dev = subsys_find_device_by_id(&memory_subsys, block_id, NULL);
-	return dev ? to_memory_block(dev) : NULL;
+	mem = radix_tree_lookup(&memory_blocks, block_id);
+	if (mem)
+		get_device(&mem->dev);
+	return mem;
 }
 
-/*
- * For now, we have a linear search to go find the appropriate
- * memory_block corresponding to a particular phys_index. If
- * this gets to be a real problem, we can always use a radix
- * tree or something here.
- *
- * This could be made generic for all device subsystems.
- */
 struct memory_block *find_memory_block(struct mem_section *section)
 {
 	unsigned long block_id = base_memory_block_id(__section_nr(section));
@@ -645,9 +647,15 @@ int register_memory(struct memory_block *memory)
 	memory->dev.offline = memory->state == MEM_OFFLINE;
 
 	ret = device_register(&memory->dev);
-	if (ret)
+	if (ret) {
 		put_device(&memory->dev);
-
+		return ret;
+	}
+	ret = radix_tree_insert(&memory_blocks, memory->dev.id, memory);
+	if (ret) {
+		put_device(&memory->dev);
+		device_unregister(&memory->dev);
+	}
 	return ret;
 }
 
@@ -705,6 +713,8 @@ static void unregister_memory(struct memory_block *memory)
 	if (WARN_ON_ONCE(memory->dev.bus != &memory_subsys))
 		return;
 
+	WARN_ON(radix_tree_delete(&memory_blocks, memory->dev.id) == NULL);
+
 	/* drop the ref. we got via find_memory_block() */
 	put_device(&memory->dev);
 	device_unregister(&memory->dev);
-- 
2.23.0