memcg: store node_id instead of pglist_data pointer

memcg: store node_id instead of pglist_data pointer

Patch series "memcg: shrink obj_stock_pcp and cache multiple objcgs", v3.

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 a single cached_objcg
pointer -- 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.

Commit d0211878ce06 ("memcg: cache obj_stock by memcg, not by objcg
pointer") landed as a temporary fix by treating sibling per-node objcgs as
equivalent for the cache lookup, intended to be reverted once per-node
kmem accounting is introduced.  This series takes a more general approach:
cache multiple objcgs per CPU using the multi-slot pattern memcg_stock_pcp
already uses, so the per-node objcg variants of one memcg can all coexist
in the stock without ever forcing a drain.  The temporary fix can then be
reverted.

To avoid increasing the per-CPU cache footprint, the first three patches
shrink the existing single-slot obj_stock_pcp fields.  The final patch
converts cached_objcg and nr_bytes into NR_OBJ_STOCK=5 slot arrays and
reorders the struct so the entire consume/refill/account hot path fits
within a single 64-byte cache line on non-debug 64-bit builds (verified
with pahole).

This patch (of 4):

The struct obj_stock_pcp stores a pointer to pglist_data for the slab
stats cached on the cpu.  On 64-bit machines, this costs 8 bytes.  The
pointer is not strictly required: NODE_DATA() can recover it from the node
id.  Replace cached_pgdat with int16_t node_id and use NUMA_NO_NODE as the
"no stats cached" sentinel.

At the moment all the archs limit MAX_NUMNODES to 1024 so int16_t is
plenty; a BUILD_BUG_ON() makes sure we notice if that ever changes.

Link: https://lore.kernel.org/20260526033931.1760588-1-shakeel.butt@linux.dev
Link: https://lore.kernel.org/20260526033931.1760588-2-shakeel.butt@linux.dev
Fixes: 01b9da291c49 ("mm: memcontrol: convert objcg to be per-memcg per-node type")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Tested-by: kernel test robot <oliver.sang@intel.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Acked-by: Qi Zheng <qi.zheng@linux.dev>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>