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

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Date:   Thu, 16 Jan 2020 12:17:01 -0500
From:   Don Dutile <ddutile@...hat.com>
To:     David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...nel.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        nathanl@...ux.ibm.com, ricklind@...ux.vnet.ibm.com,
        Scott Cheloha <cheloha@...ux.ibm.com>
Subject: Re: [PATCH v4] drivers/base/memory.c: cache blocks in radix tree to
 accelerate lookup

On 1/16/20 10:28 AM, David Hildenbrand wrote:
> On 16.01.20 16:22, Michal Hocko wrote:
>> On Wed 15-01-20 20:09:48, David Hildenbrand wrote:
>>> On 09.01.20 22:25, Scott Cheloha 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.
>>>>
>>>> Signed-off-by: Scott Cheloha <cheloha@...ux.ibm.com>
>>>> Acked-by: David Hildenbrand <david@...hat.com>
>>>> Acked-by: Nathan Lynch <nathanl@...ux.ibm.com>
>>>> Acked-by: Michal Hocko <mhocko@...e.com>
>>>
>>> Soooo,
>>>
>>> I just learned that radix trees are nowadays only a wrapper for xarray
>>> (for quite a while already!), and that the xarray interface shall be
>>> used in new code.
>>
>> Good point. I somehow didn't realize this would add more work for a
>> later code refactoring. The mapping should be pretty straightforward.
> 
> Yes it is. @Scott, care to send a fixup that does the mapping?
> 
>>
>> Thanks for noticing!
> 
> Don noticed it, so thanks to him :)
> 
Backporting XArray to RHEL-8, and continuing to support radix-tree api in RHEL-8 due to RHEL rulz, it wasn't stable/bug-free everyewhere, etc., was a challenge, thus my 'sensitivity' to seeing new radix-tree code upstream.

>