[PATCH] mm/percpu: account full object size for percpu memory allocations

["Harry Yoo (Meta)" <harry@kernel.org>]

When memory allocation profiling is enabled, the pages backing percpu
memory chunks are attributed to func:pcpu_alloc_pages. However, the
accounting of individual percpu memory allocations via alloc_percpu()
and friends record only the size of a single object rather than its
full size across all CPUs.

As a result, it is often not straightforward to identify which kernel
code is actually responsible for percpu memory, because the size is
under-accounted by a factor of num_possible_cpus().

Account full object size with pcpu_obj_full_size(size) instead, which
multiplies the object size by num_possible_cpus() and adds the objcg
pointer overhead. This is consistent with what memcg already does for
charging percpu allocations.

Verified with 64-CPU VM by spawning 400 processes in a memory cgroup and
measuring mm_init()'s rss_stat counters.

Delta of kernel/fork.c:1142 func:mm_init in /proc/allocinfo:

                  bytes   calls   per allocation
    unpatched      6320     395         16
    patched      418176     396       1056

'percpu' field in memory.stat: 422400 (1056 bytes per process)

Memory allocation profiling now accounts the same amount of memory as
what memcg charges.

Cc: stable@vger.kernel.org
Fixes: 60fa4a9e2323 ("mm: percpu: add codetag reference into pcpuobj_ext")
Assisted-by: LLM # Verification
Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>
---
 mm/percpu.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 3eff382e565a..d7883507dc15 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1696,14 +1696,15 @@ static void pcpu_alloc_tag_alloc_hook(struct pcpu_chunk *chunk, int off,
 {
 	if (mem_alloc_profiling_enabled() && likely(chunk->obj_exts)) {
 		alloc_tag_add(&chunk->obj_exts[off >> PCPU_MIN_ALLOC_SHIFT].tag,
-			      current->alloc_tag, size);
+			      current->alloc_tag, pcpu_obj_full_size(size));
 	}
 }
 
 static void pcpu_alloc_tag_free_hook(struct pcpu_chunk *chunk, int off, size_t size)
 {
 	if (mem_alloc_profiling_enabled() && likely(chunk->obj_exts))
-		alloc_tag_sub(&chunk->obj_exts[off >> PCPU_MIN_ALLOC_SHIFT].tag, size);
+		alloc_tag_sub(&chunk->obj_exts[off >> PCPU_MIN_ALLOC_SHIFT].tag,
+			      pcpu_obj_full_size(size));
 }
 #else
 static void pcpu_alloc_tag_alloc_hook(struct pcpu_chunk *chunk, int off,

---
base-commit: 0e199ae6e803e76ebf128e009d336e5917221e63
change-id: 20261013-fix-alloc-percpu-accounting-48bf24919caf

Best regards,
--  
Cheers,
Harry / Hyeonggon