Re: [PATCH v5 1/2] drm/buddy: Optimize free block management with RB tree

[Arunpravin Paneer Selvam <arunpravin.paneerselvam@amd.com>]

On 9/2/2025 3:53 PM, Jani Nikula wrote:
> On Tue, 02 Sep 2025, Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com> wrote:
>> Replace the freelist (O(n)) used for free block management with a
>> red-black tree, providing more efficient O(log n) search, insert,
>> and delete operations. This improves scalability and performance
>> when managing large numbers of free blocks per order (e.g., hundreds
>> or thousands).
>>
>> In the VK-CTS memory stress subtest, the buddy manager merges
>> fragmented memory and inserts freed blocks into the freelist. Since
>> freelist insertion is O(n), this becomes a bottleneck as fragmentation
>> increases. Benchmarking shows list_insert_sorted() consumes ~52.69% CPU
>> with the freelist, compared to just 0.03% with the RB tree
>> (rbtree_insert.isra.0), despite performing the same sorted insert.
>>
>> This also improves performance in heavily fragmented workloads,
>> such as games or graphics tests that stress memory.
>>
>> v3(Matthew):
>>    - Remove RB_EMPTY_NODE check in force_merge function.
>>    - Rename rb for loop macros to have less generic names and move to
>>      .c file.
>>    - Make the rb node rb and link field as union.
>>
>> v4(Jani Nikula):
>>    - The kernel-doc comment should be "/**"
>>    - Move all the rbtree macros to rbtree.h and add parens to ensure
>>      correct precedence.
>>
>> Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
>> ---
>>   drivers/gpu/drm/drm_buddy.c | 142 ++++++++++++++++++++++--------------
>>   include/drm/drm_buddy.h     |   9 ++-
>>   include/linux/rbtree.h      |  56 ++++++++++++++
>>   3 files changed, 152 insertions(+), 55 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
>> index a94061f373de..978cabfbcf0f 100644
>> --- a/drivers/gpu/drm/drm_buddy.c
>> +++ b/drivers/gpu/drm/drm_buddy.c
> ...
>
>> +static inline struct drm_buddy_block *
>> +rbtree_last_entry(struct drm_buddy *mm, unsigned int order)
> Drive-by reminder that "inline" in a .c file is, in absense of evidence
> to the contrary, superfluous. Please just let the compiler do its job.
Ah, I missed taking out the inline. Thanks for catching that.

Thanks,
Arun.
>
> BR,
> Jani.
>
>