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

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Message-Id: <A7C7E3ED-3A02-43C7-B739-53A7756822D4@redhat.com>
Date:   Thu, 9 Jan 2020 23:17:09 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Scott Cheloha <cheloha@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Hildenbrand <david@...hat.com>, nathanl@...ux.ibm.com,
        ricklind@...ux.vnet.ibm.com, mhocko@...e.com,
        Scott Cheloha <cheloha@...ux.ibm.com>
Subject: Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to accelerate lookup



> Am 09.01.2020 um 23:00 schrieb Andrew Morton <akpm@...ux-foundation.org>:
> 
> On Thu,  9 Jan 2020 15:25:16 -0600 Scott Cheloha <cheloha@...ux.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).