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

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Message-ID: <5dac8dc2-08a8-394f-d464-433d74ec62c1@redhat.com>
Date:   Thu, 9 Jan 2020 10:24:00 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org,
        Scott Cheloha <cheloha@...ux.vnet.ibm.com>,
        nathanl@...ux.ibm.com, ricklind@...ux.vnet.ibm.com,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] drivers/base/memory.c: cache blocks in radix tree to
 accelerate lookup

On 09.01.20 10:19, Michal Hocko wrote:
> On Thu 09-01-20 09:56:23, Greg KH wrote:
>> On Thu, Jan 09, 2020 at 09:49:55AM +0100, Michal Hocko wrote:
>>> On Tue 07-01-20 22:48:04, Michal Hocko wrote:
>>>> [Cc Andrew]
>>>>
>>>> On Tue 17-12-19 13:32:38, Scott Cheloha wrote:
>>>>> 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.
>>>>
>>>> Noting that this is O(N^2) would be useful.
>>>>
>>>>> Lookup is much faster if we cache the blocks in a radix tree.
>>>>
>>>> While this is really easy and straightforward, is there any reason why
>>>> subsys_find_device_by_id has to use such a slow lookup? I suspect nobody
>>>> simply needed a more optimized data structure for that purpose yet.
>>>> Would it be too hard to use radix tree for all lookups rather than
>>>> adding a shadow copy for memblocks?
>>>
>>> Greg, Rafael, this seems to be your domain. Do you have any opinion on
>>> this?
>>
>> No one has cared about the speed of that call as it has never been on
>> any "fast path" that I know of.  And it should just be O(N), isn't it
>> just walking the list of devices in order?
> 
> Which means that if you have to call it N times then it is O(N^2) and
> that is the case here because you are adding N memblocks. See
> memory_dev_init
>   for each memblock
>     add_memory_block
>       init_memory_block
>         find_memory_block_by_id # checks all existing devices
>         register_memory
> 	  device_register # add new device
>   
> In this particular case find_memory_block_by_id is called mostly to make
> sure we are no re-registering something multiple times which shouldn't
> happen so it sucks to spend a lot of time on that. We might think of
> removing that for boot time but who knows what kind of surprises we
> might see from crazy HW setups.
>  
>> If the "memory subsystem" wants a faster lookup for their objects,
>> there's nothing stopping you from using your own data structure for the
>> pointers to the objects if you want.  Just be careful about the lifetime
>> rules.
> 
> The main question is whether replacing the linked list with a radix tree
> in the generic code is something more meaningful.
> 

Please note that there are a total of 2 (!) users of
subsys_find_device_by_id() only ... makes me wonder if we should get rid
of  subsys_find_device_by_id() instead and handle it only in the caller
directly. Rewriting the core (list->tree with all the locking logic)
sounds to me like an unnecessary rework.

-- 
Thanks,

David / dhildenb