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

[Harry Yoo <harry@kernel.org>]

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