Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup

David Hildenbrand
2020-01-09T22:17:16Z

> Am 09.01.2020 um 23:00 schrieb Andrew Morton <akpm@linux-foundation.org>:
>
> ﻿On Thu, 9 Jan 2020 15:25:16 -0600 Scott Cheloha <cheloha@linux.vnet.ibm.com> wrote:
>
>> Searching for a particular memory block by id is an O(n) operation
>> because each memory block's underlying device is kept in an unsorted
>> linked list on the subsystem bus.
>>
>> We can cut the lookup cost to O(log n) if we cache the memory blocks in
>> a radix tree. With a radix tree cache in place both memory subsystem
>> initialization and memory hotplug run palpably faster on systems with a
>> large number of memory blocks.
>>
>> ...
>>
>> @@ -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);
>
> What protects this tree from racy accesses?

I think the device hotplug lock currently (except during boot where no races can happen).