Re: [PATCH v3 4/4] memcg: multi objcg charge support

[Harry Yoo <harry@kernel.org>]

On 5/26/26 12:39 PM, Shakeel Butt wrote:
> Commit 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg
> per-node type") split a memcg's single obj_cgroup into one per NUMA
> node so that reparenting LRU folios can take per-node lru locks. As a
> side effect, the per-CPU obj_stock_pcp -- which caches exactly one
> cached_objcg -- thrashes on workloads where threads of the same memcg
> run on different NUMA nodes. The kernel test robot reported a 67.7%
> regression on stress-ng.switch.ops_per_sec from this pattern.
> 
> Mirror the multi-slot pattern already used by memcg_stock_pcp: turn
> nr_bytes and cached_objcg into NR_OBJ_STOCK-element arrays, scan all
> slots on consume/refill/account, prefer empty slots when inserting,
> and evict a random slot only when full. With multiple slots a CPU can
> hold the per-node objcg variants of one memcg plus a few siblings
> without ever forcing a drain.

nit: evict a random slot -> evict a slot round-robin
> A single int8_t index records which slot the cached slab stats belong
> to; the stats are flushed on slot or pgdat change. With NR_OBJ_STOCK
> = 5 the layout (verified with pahole) is:
> 
>    offset 0  : lock(1) + index(1) + node_id(2) + slab stats(4) = 8B
>    offset 8  : nr_bytes[5]                                     = 10B
>    offset 18 : padding                                         = 6B
>    offset 24 : cached[5]                                       = 40B
>    offset 64 : (line 2) work_struct + flags (cold)
> 
> so consume_obj_stock, refill_obj_stock and the slab account path each
> touch exactly one 64-byte cache line on non-debug 64-bit builds.
> 
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202605121641.b6a60cb0-lkp@intel.com
> Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
> Tested-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---

Looks good to me,
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>

-- 
Cheers,
Harry / Hyeonggon