Re: [PATCH v3 2/3] mm/memcg: use kmem_cache when alloc memcg

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 25, 2025 at 11:19:24AM +0800, Huan Yang wrote:
> When tracing mem_cgroup_alloc() with kmalloc ftrace, we observe:
> 
> kmalloc: call_site=mem_cgroup_css_alloc+0xd8/0x5b4 ptr=000000003e4c3799
>     bytes_req=2312 bytes_alloc=4096 gfp_flags=GFP_KERNEL|__GFP_ZERO node=-1
>     accounted=false
> 
> The output indicates that while allocating mem_cgroup struct (2312 bytes),
> the slab allocator actually provides 4096-byte chunks. This occurs because:
> 
> 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 allocation
> 
> 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+0xbc/0x5d4
>     ptr=00000000695c1806 bytes_req=2312 bytes_alloc=2368
>     gfp_flags=GFP_KERNEL|__GFP_ZERO node=-1 accounted=false
> 
> Each memcg alloc offer 2368bytes(include hw cacheline align), compare to
> 4096, avoid waste.
> 
> Signed-off-by: Huan Yang <link@vivo.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>