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

[Jani Nikula <jani.nikula@linux.intel.com>]

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.

BR,
Jani.


-- 
Jani Nikula, Intel