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mm/percpu: account full object size for percpu memory allocations

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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.

Link: https://lore.kernel.org/20261013-fix-alloc-percpu-accounting-v1-1-03e788a1e41f@kernel.org
Fixes: 60fa4a9e23 ("mm: percpu: add codetag reference into pcpuobj_ext")
Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Assisted-by: LLM # Verification
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Hao Ge <hao.ge@linux.dev>
Cc: <stable@vger.kernel.org>
This commit is contained in:
Harry Yoo (Meta) 2026-10-13 20:34:39 +01:00 committed by Andrew Morton
parent 454143aeb4
commit c7a467fbcb
1 changed files with 3 additions and 2 deletions
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5
mm/percpu.c
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@ -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,