Re: [PATCH v2] mm/percpu, memcontrol: Per-memcg-lruvec percpu accounting

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

On Fri, Apr 03, 2026 at 08:38:43PM -0700, Joshua Hahn wrote:
> enum memcg_stat_item includes memory that is tracked on a per-memcg
> level, but not at a per-node (and per-lruvec) level. Diagnosing
> memory pressure for memcgs in multi-NUMA systems can be difficult,
> since not all of the memory accounted in memcg can be traced back
> to a node. In scenarios where numa nodes in an memcg are asymmetrically
> stressed, this difference can be invisible to the user.
> 
> Convert MEMCG_PERCPU_B from a memcg_stat_item to a memcg_node_stat_item
> to give visibility into per-node breakdowns for percpu allocations.
> 
> This will get us closer to being able to know the memcg and physical
> association of all memory on the system. Specifically for percpu, this
> granularity will help demonstrate footprint differences on systems with
> asymmetric NUMA nodes.
> 
> Because percpu memory is accounted at a sub-PAGE_SIZE level, we must
> account node level statistics (accounted in PAGE_SIZE units) and
> memcg-lruvec statistics separately. Account node statistics when the pcpu
> pages are allocated, and account memcg-lruvec statistics when pcpu
> objects are handed out.
> 
> To do account these separately, expose mod_memcg_lruvec_state to be
> used outside of memcontrol.
> 
> The memory overhead of this patch is small; it adds 16 bytes
> per-cgroup-node-cpu. For an example machine with 200 CPUs split across
> 2 nodes and 50 cgroups in the system, we see a 312.5 kB increase. Note
> that this is the same cost as any other item in memcg_node_stat_item.
> 
> Performance impact is also negligible. These are results from a kernel
> module which performs 100k percpu allocations via __alloc_percpu_gfp
> with GFP_KERNEL | __GFP_ACCOUNT in a cgroup, across 20 trials.
> Batched performs 100k allocations followed by 100k frees, while
> interleaved performs allocation --> free --> allocation ...
> 
> +-------------+----------------+--------------+--------------+
> |    Test     | linus-upstream |    patch     |     diff     |
> +-------------+----------------+--------------+--------------+
> | Batched     | 6586 +/- 51    | 6595 +/- 35  | +9 (0.13%)   |
> | Interleaved | 1053 +/- 126   | 1085 +/- 113 | +32 (+0.85%) |
> +-------------+----------------+--------------+--------------+
> 
> One functional change is that there can be a tiny inconsistency between
> the size of the allocation used for memcg limit checking and what is
> charged to each lruvec due to dropping fractional charges when rounding.
> In reality this value is very very small and always lies on the side of
> memory checking at a higher threshold, so there is no behavioral change
> from userspace.
> 
> Signed-off-by: Joshua Hahn <joshua.hahnjy@gmail.com>
> ---
>  include/linux/memcontrol.h |  4 +++-
>  include/linux/mmzone.h     |  4 +++-
>  mm/memcontrol.c            | 12 +++++-----
>  mm/percpu-vm.c             | 14 ++++++++++--
>  mm/percpu.c                | 45 ++++++++++++++++++++++++++++++++++----
>  mm/vmstat.c                |  1 +
>  6 files changed, 66 insertions(+), 14 deletions(-)
> 
> diff --git a/mm/percpu-vm.c b/mm/percpu-vm.c
> index 4f5937090590d..e36b639f521dd 100644
> --- a/mm/percpu-vm.c
> +++ b/mm/percpu-vm.c
> @@ -65,6 +66,10 @@ static void pcpu_free_pages(struct pcpu_chunk *chunk,
>  				__free_page(page);
>  		}
>  	}
> +
> +	for_each_node(nid)
> +		mod_node_page_state(NODE_DATA(nid), NR_PERCPU_B,
> +				-1L * nr_pages * nr_cpus_node(nid) * PAGE_SIZE);

Can this end up with mis-accounting due to CPU hotplug?

>  }
>  
>  /**
> @@ -84,7 +89,8 @@ static int pcpu_alloc_pages(struct pcpu_chunk *chunk,
>  			    gfp_t gfp)
>  {
>  	unsigned int cpu, tcpu;
> -	int i;
> +	int nr_pages = page_end - page_start;
> +	int i, nid;
>  
>  	gfp |= __GFP_HIGHMEM;
>  
> @@ -97,6 +103,10 @@ static int pcpu_alloc_pages(struct pcpu_chunk *chunk,
>  				goto err;
>  		}
>  	}
> +
> +	for_each_node(nid)
> +		mod_node_page_state(NODE_DATA(nid), NR_PERCPU_B,
> +				    nr_pages * nr_cpus_node(nid) * PAGE_SIZE);
>  	return 0;
>  
>  err:

-- 
Cheers,
Harry / Hyeonggon