[PATCH v2 2/3] mm/memcg: use kmem_cache when alloc memcg pernode info

[Huan Yang]

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 4KB 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 (2312 bytes) 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 2944 bytes(include hw cacheline align),
compare to 4KB, it avoid waste.

Signed-off-by: Huan Yang <link@xxxxxxxx>
---
 mm/memcontrol.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index cb32a498e5ae..e8797382aeb4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -96,6 +96,7 @@ 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);
@@ -3601,7 +3602,10 @@ 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 = likely(memcg_pn_cachep) ?
+		     kmem_cache_alloc_node(memcg_pn_cachep,
+					   GFP_KERNEL | __GFP_ZERO, node) :
+		     kzalloc_node(sizeof(*pn), GFP_KERNEL, node);
 	if (!pn)
 		return false;

@@ -5065,6 +5069,9 @@ static 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;
 }
 subsys_initcall(mem_cgroup_init);
--
2.48.1