mm/memcg: use kmem_cache when alloc memcg pernode info

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.

Link: https://lkml.kernel.org/r/20250425031935.76411-4-link@vivo.com
Signed-off-by: Huan Yang <link@vivo.com>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Francesco Valla <francesco@valla.it>
Cc: guoweikang <guoweikang.kernel@gmail.com>
Cc: Huang Shijie <shijie@os.amperecomputing.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: "Paul E . McKenney" <paulmck@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Raul E Rangel <rrangel@chromium.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6d713fe10221f12e16c0976f3680a3b71bfb9503..8ed265852423f391731609ee73b63ad9d66c65b6 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -97,6 +97,7 @@ static bool cgroup_memory_nokmem __ro_after_init;
 static bool cgroup_memory_nobpf __ro_after_init;
 
 static struct kmem_cache *memcg_cachep;
+static struct kmem_cache *memcg_pn_cachep;
 
 #ifdef CONFIG_CGROUP_WRITEBACK
 static DECLARE_WAIT_QUEUE_HEAD(memcg_cgwb_frn_waitq);
@@ -3614,7 +3615,8 @@ static bool alloc_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
 {
        struct mem_cgroup_per_node *pn;
 
-       pn = kzalloc_node(sizeof(*pn), GFP_KERNEL, node);
+       pn = kmem_cache_alloc_node(memcg_pn_cachep, GFP_KERNEL | __GFP_ZERO,
+                                  node);
        if (!pn)
                return false;
 
@@ -5075,6 +5077,9 @@ int __init mem_cgroup_init(void)
        memcg_cachep = kmem_cache_create("mem_cgroup", memcg_size, 0,
                                         SLAB_PANIC | SLAB_HWCACHE_ALIGN, NULL);
 
+       memcg_pn_cachep = KMEM_CACHE(mem_cgroup_per_node,
+                                    SLAB_PANIC | SLAB_HWCACHE_ALIGN);
+
        return 0;
 }