[PATCH v3] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

[Scott Cheloha]

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@xxxxxxxxxxxxxxxxxx>
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
---
v2 incorporates suggestions from David Hildenbrand.

v3 changes:
  - Rebase atop "drivers/base/memory.c: drop the mem_sysfs_mutex"

  - Be conservative: don't use radix_tree_for_each_slot() in
    walk_memory_blocks() yet.  It introduces RCU which could
    change behavior.  Walking the tree "by hand" with
    find_memory_block_by_id() is slower but keeps the patch
    simple.

 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 799b43191dea..8902930d5ef2 100644
--- a/drivers/base/memory.c
+++ b/drivers/base/memory.c
@@ -19,6 +19,7 @@
 #include <linux/memory.h>
 #include <linux/memory_hotplug.h>
 #include <linux/mm.h>
+#include <linux/radix-tree.h>
 #include <linux/stat.h>
 #include <linux/slab.h>
 
@@ -56,6 +57,13 @@ 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)
@@ -572,20 +580,14 @@ 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));
@@ -628,9 +630,15 @@ 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;
 }
 
@@ -688,6 +696,8 @@ 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);
-- 
2.24.0