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

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Message-ID: <20250427115647.GB116315@cmpxchg.org>
Date: Sun, 27 Apr 2025 07:56:47 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Huan Yang <link@...o.com>
Cc: Michal Hocko <mhocko@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...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 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@...o.com>

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