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

David Hildenbrand
2020-01-09T22:35:52Z

> Am 09.01.2020 um 23:28 schrieb Andrew Morton <akpm@linux-foundation.org>:
>
> ﻿On Thu, 9 Jan 2020 23:17:09 +0100 David Hildenbrand <david@redhat.com> wrote:
>
>>
>>
>>>> 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).
>>
>
> So this?
>
> --- a/drivers/base/memory.c~drivers-base-memoryc-cache-blocks-in-radix-tree-to-accelerate-lookup-fix
> +++ a/drivers/base/memory.c
> @@ -61,6 +61,9 @@ static struct bus_type memory_subsys = {
> * Memory blocks are cached in a local radix tree to avoid
> * a costly linear search for the corresponding device on
> * the subsystem bus.
> + *
> + * Protected by mem_hotplug_lock in mem_hotplug_begin(), and by the guaranteed
> + * single-threadness at boot time.
> */
> static RADIX_TREE(memory_blocks, GFP_KERNEL);
>
>
> But are we sure this is all true?

I think the device hotplug lock, not the memory hotplug lock. Will double check later.
>