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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx

Why cc:stable?