linux-kernel - Re: [PATCH v3 2/3] mm/memcg: use kmem

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Message-ID: <CAGj-7pW2Dy4U=x_MU8D+-o+JFLrQfO=b4tXD-J4B=ocHrv6fmA@mail.gmail.com>
Date: Thu, 24 Apr 2025 21:12:29 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Huan Yang <link@...o.com>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Muchun Song <muchun.song@...ux.dev>, 
	Andrew Morton <akpm@...ux-foundation.org>, Petr Mladek <pmladek@...e.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Francesco Valla <francesco@...la.it>, Raul E Rangel <rrangel@...omium.org>, 
	"Paul E. McKenney" <paulmck@...nel.org>, Huang Shijie <shijie@...amperecomputing.com>, 
	Guo Weikang <guoweikang.kernel@...il.com>, "Uladzislau Rezki (Sony)" <urezki@...il.com>, 
	KP Singh <kpsingh@...nel.org>, cgroups@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH v3 2/3] mm/memcg: use kmem_cache when alloc memcg

On Thu, Apr 24, 2025 at 8:19 PM Huan Yang <link@...o.com> 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@...o.com>

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