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

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

[Harry Yoo (Meta)]

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

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

[Andrew Morton]

On Tue, 13 Oct 2026 20:34:39 +0100 "Harry Yoo (Meta)" <harry@kernel.org> wrote:

> 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:

Please make it use mm_init() rather than func:mm_init.

> 
>                   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.

Fair enough.

> Cc: stable@vger.kernel.org

Why cc:stable?

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

[Harry Yoo]

On Tue, Oct 13, 2026 at 02:54:24PM -0700, Andrew Morton wrote:
> On Tue, 13 Oct 2026 20:34:39 +0100 "Harry Yoo (Meta)" <harry@kernel.org> wrote:
> 
> > 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:
> 
> Please make it use mm_init() rather than func:mm_init.

This is the output format of /proc/allocinfo:
https://docs.kernel.org/filesystems/proc.html#allocinfo
> Example output.
> 
>  > tail -n +3 /proc/allocinfo | sort -rn
> 127664128    31168 mm/page_ext.c:270 func:alloc_page_ext
>  56373248     4737 mm/slub.c:2259 func:alloc_slab_page
>  14880768     3633 mm/readahead.c:247 func:page_cache_ra_unbounded

> > 
> >                   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.
> 
> Fair enough.
> 
> > Cc: stable@vger.kernel.org
> 
> Why cc:stable?

For anyone who uses memory allocation profiling to understand which
code locations allocated how much memory, it gives a false impression
that certain perpcu memory allocations are not as big as they actually
are.

On some workloads percpu memory (func:pcpu_alloc_pages) shows up as one
of the biggest memory allocations, but the actual users of percpu
memory allocator don't show up as they are under-reported.

-- 
Cheers,
Harry / Hyeonggon

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

[Andrew Morton]

On Tue, 13 Oct 2026 23:23:17 +0100 Harry Yoo <harry@kernel.org> wrote:

> On Tue, Oct 13, 2026 at 02:54:24PM -0700, Andrew Morton wrote:
> > On Tue, 13 Oct 2026 20:34:39 +0100 "Harry Yoo (Meta)" <harry@kernel.org> wrote:
> > 
> > > 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:
> > 
> > Please make it use mm_init() rather than func:mm_init.
> 
> This is the output format of /proc/allocinfo:

oh, OK.

> https://docs.kernel.org/filesystems/proc.html#allocinfo
> > Example output.
> > 
> >  > tail -n +3 /proc/allocinfo | sort -rn
> > 127664128    31168 mm/page_ext.c:270 func:alloc_page_ext
> >  56373248     4737 mm/slub.c:2259 func:alloc_slab_page
> >  14880768     3633 mm/readahead.c:247 func:page_cache_ra_unbounded

Well that's odd.  What can a caller be other than a function?

> > 
> > Why cc:stable?
> 
> For anyone who uses memory allocation profiling to understand which
> code locations allocated how much memory, it gives a false impression
> that certain perpcu memory allocations are not as big as they actually
> are.
> 
> On some workloads percpu memory (func:pcpu_alloc_pages) shows up as one
> of the biggest memory allocations, but the actual users of percpu
> memory allocator don't show up as they are under-reported.

OK.  I guess it's a small deal, but a larger deal for allocinfo users!