[PATCH v3 0/3] Use kmem_cache for memcg alloc

[Huan Yang]

The mem_cgroup_alloc function creates mem_cgroup struct and it's associated
structures including mem_cgroup_per_node.
Through detailed analysis on our test machine (Arm64, 16GB RAM, 6.6 kernel,
1 NUMA node, memcgv2 with nokmem,nosocket,cgroup_disable=pressure),
we can observe the memory allocation for these structures using the
following shell commands:
  # Enable tracing
  echo 1 > /sys/kernel/tracing/events/kmem/kmalloc/enable
  echo 1 > /sys/kernel/tracing/tracing_on
  cat /sys/kernel/tracing/trace_pipe | grep kmalloc | grep mem_cgroup

  # Trigger allocation if cgroup subtree do not enable memcg
  echo +memory > /sys/fs/cgroup/cgroup.subtree_control

Ftrace Output:
  # mem_cgroup struct allocation
  sh-6312    [000] ..... 58015.698365: 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

  # mem_cgroup_per_node allocation
  sh-6312    [000] ..... 58015.698389: 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

Key Observations:
  1. Both structures use kmalloc with requested sizes between 2KB-4KB
  2. Allocation alignment forces 4KB slab usage due to pre-defined sizes
     (64B, 128B,..., 2KB, 4KB, 8KB)
  3. Memory waste per memcg instance:
      Base struct: 4096 - 2312 = 1784 bytes
      Per-node struct: 4096 - 2896 = 1200 bytes
      Total waste: 2984 bytes (1-node system)
      NUMA scaling: (1200 + 8) * nr_node_ids bytes
So, it's a little waste.

This patchset introduces dedicated kmem_cache:
  Patch2 - mem_cgroup kmem_cache - memcg_cachep
  Patch3 - mem_cgroup_per_node kmem_cache - memcg_pn_cachep

The benefits of this change can be observed with the following tracing
commands:
  # Enable tracing
  echo 1 > /sys/kernel/tracing/events/kmem/kmem_cache_alloc/enable
  echo 1 > /sys/kernel/tracing/tracing_on
  cat /sys/kernel/tracing/trace_pipe | grep kmem_cache_alloc | grep mem_cgroup
  # In another terminal:
  echo +memory > /sys/fs/cgroup/cgroup.subtree_control


The output might now look like this:

  # mem_cgroup struct allocation
  sh-9827     [000] .....   289.513598: 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
  # mem_cgroup_per_node allocation
  sh-9827     [000] .....   289.513602: 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

This indicates that the `mem_cgroup` struct now requests 2312 bytes
and is allocated 2368 bytes, while `mem_cgroup_per_node` requests 2896 bytes
and is allocated 2944 bytes.
The slight increase in allocated size is due to `SLAB_HWCACHE_ALIGN` in the
`kmem_cache`.

Without `SLAB_HWCACHE_ALIGN`, the allocation might appear as:

  # mem_cgroup struct allocation
  sh-9269     [003] .....    80.396366: kmem_cache_alloc:
    call_site=mem_cgroup_css_alloc+0xbc/0x5d4 ptr=000000005b12b475
    bytes_req=2312 bytes_alloc=2312 gfp_flags=GFP_KERNEL|__GFP_ZERO node=-1
    accounted=false

  # mem_cgroup_per_node allocation
  sh-9269     [003] .....    80.396411: kmem_cache_alloc:
    call_site=mem_cgroup_css_alloc+0x1b8/0x5d4 ptr=00000000f347adc6
    bytes_req=2896 bytes_alloc=2896 gfp_flags=GFP_KERNEL|__GFP_ZERO node=0
    accounted=false

While the `bytes_alloc` now matches the `bytes_req`, this patchset defaults
to using `SLAB_HWCACHE_ALIGN` as it is generally considered more beneficial
for performance. Please let me know if there are any issues or if I've
misunderstood anything.

This patchset also move mem_cgroup_init ahead of cgroup_init() due to
cgroup_init() will allocate root_mem_cgroup, but each initcall invoke after
cgroup_init, so if each kmem_cache do not prepare, we need testing NULL before
use it.

ChangeLog:
 v2 -> v3:
   Move v2 patch3 ahead, reuse and move mem_cgroup_init ahead of cgroup_init.
 v1 -> v2:
   Patch1-2 simple change commit message.
   Patch3: Add mem_cgroup_init_early to help "memcg" prepare resources
           before cgroup_init().

v2: https://lore.kernel.org/all/20250424120937.96164-1-link@xxxxxxxx/
v1: https://lore.kernel.org/all/20250423084306.65706-1-link@xxxxxxxx/


Huan Yang (3):
  mm/memcg: move mem_cgroup_init() ahead of cgroup_init()
  mm/memcg: use kmem_cache when alloc memcg
  mm/memcg: use kmem_cache when alloc memcg pernode info

 include/linux/memcontrol.h |  3 +++
 init/main.c                |  2 ++
 mm/memcontrol.c            | 21 ++++++++++++++++-----
 3 files changed, 21 insertions(+), 5 deletions(-)


base-commit: 2c9c612abeb38aab0e87d48496de6fd6daafb00b
--
2.48.1