Re: [PATCH v3 3/3] mm/memcg: use kmem_cache when alloc memcg pernode info

[Shakeel Butt]

On Thu, Apr 24, 2025 at 8:19 PM Huan Yang <link@xxxxxxxx> wrote:
>
> When tracing mem_cgroup_per_node allocations with kmalloc ftrace:
>
> kmalloc: call_site=mem_cgroup_css_alloc+0x1d8/0x5b4 ptr=00000000d798700c
>     bytes_req=2896 bytes_alloc=4096 gfp_flags=GFP_KERNEL|__GFP_ZERO node=0
>     accounted=false
>
> This reveals the slab allocator provides 4096B chunks for 2896B
> mem_cgroup_per_node due to:
>
> 1. The slab allocator predefines bucket sizes from 64B to 8096B
> 2. The mem_cgroup allocation size (2312B) falls between the 2KB and 4KB
>    slabs
> 3. The allocator rounds up to the nearest larger slab (4KB), resulting in
>    ~1KB wasted memory per memcg alloc - per node.
>
> This patch introduces a dedicated kmem_cache for mem_cgroup structs,
> achieving precise memory allocation. Post-patch ftrace verification shows:
>
> kmem_cache_alloc: call_site=mem_cgroup_css_alloc+0x1b8/0x5d4
>     ptr=000000002989e63a bytes_req=2896 bytes_alloc=2944
>     gfp_flags=GFP_KERNEL|__GFP_ZERO node=0 accounted=false
>
> Each mem_cgroup_per_node alloc 2944bytes(include hw cacheline align),
> compare to 4096, it avoid waste.
>
> Signed-off-by: Huan Yang <link@xxxxxxxx>

Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>