Re: [PATCH mm-new v3] mm/memcontrol: Add memory.stat_refresh for on-demand stats flushing

[Harry Yoo <harry.yoo@oracle.com>]

On Mon, Nov 10, 2025 at 06:19:48PM +0800, Leon Huang Fu wrote:
> Memory cgroup statistics are updated asynchronously with periodic
> flushing to reduce overhead. The current implementation uses a flush
> threshold calculated as MEMCG_CHARGE_BATCH * num_online_cpus() for
> determining when to aggregate per-CPU memory cgroup statistics. On
> systems with high core counts, this threshold can become very large
> (e.g., 64 * 256 = 16,384 on a 256-core system), leading to stale
> statistics when userspace reads memory.stat files.
> 
> This is particularly problematic for monitoring and management tools
> that rely on reasonably fresh statistics, as they may observe data
> that is thousands of updates out of date.
> 
> Introduce a new write-only file, memory.stat_refresh, that allows
> userspace to explicitly trigger an immediate flush of memory statistics.
>
> Writing any value to this file forces a synchronous flush via
> __mem_cgroup_flush_stats(memcg, true) for the cgroup and all its
> descendants, ensuring that subsequent reads of memory.stat and
> memory.numa_stat reflect current data.
> 
> This approach follows the pattern established by /proc/sys/vm/stat_refresh
> and memory.peak, where the written value is ignored, keeping the
> interface simple and consistent with existing kernel APIs.
> 
> Usage example:
>   echo 1 > /sys/fs/cgroup/mygroup/memory.stat_refresh
>   cat /sys/fs/cgroup/mygroup/memory.stat
> 
> The feature is available in both cgroup v1 and v2 for consistency.
> 
> Signed-off-by: Leon Huang Fu <leon.huangfu@shopee.com>
> ---
> v2 -> v3:
>   - Flush stats by memory.stat_refresh (per Michal)
>   - https://lore.kernel.org/linux-mm/20251105074917.94531-1-leon.huangfu@shopee.com/
> 
> v1 -> v2:
>   - Flush stats when write the file (per Michal).
>   - https://lore.kernel.org/linux-mm/20251104031908.77313-1-leon.huangfu@shopee.com/
> 
>  Documentation/admin-guide/cgroup-v2.rst | 21 +++++++++++++++++--
>  mm/memcontrol-v1.c                      |  4 ++++
>  mm/memcontrol-v1.h                      |  2 ++
>  mm/memcontrol.c                         | 27 ++++++++++++++++++-------
>  4 files changed, 45 insertions(+), 9 deletions(-)

Hi Leon, I have a few questions on the patch.

> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 3345961c30ac..ca079932f957 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1337,7 +1337,7 @@ PAGE_SIZE multiple when read back.
>  	cgroup is within its effective low boundary, the cgroup's
>  	memory won't be reclaimed unless there is no reclaimable
>  	memory available in unprotected cgroups.
> -	Above the effective low	boundary (or
> +	Above the effective low	boundary (or

Is this whitespace change? it looks the same as before.

>  	effective min boundary if it is higher), pages are reclaimed
>  	proportionally to the overage, reducing reclaim pressure for
>  	smaller overages.
> @@ -1785,6 +1785,23 @@ The following nested keys are defined.
>  		up if hugetlb usage is accounted for in memory.current (i.e.
>  		cgroup is mounted with the memory_hugetlb_accounting option).
> 
> +  memory.stat_refresh
> +	A write-only file which exists on non-root cgroups.

Why don't we create the file for the root cgroup?

> +	Writing any value to this file forces an immediate flush of
> +	memory statistics for this cgroup and its descendants. This
> +	ensures subsequent reads of memory.stat and memory.numa_stat
> +	reflect the most current data.
> +
> +	This is useful on high-core count systems where per-CPU caching
> +	can lead to stale statistics, or when precise memory usage
> +	information is needed for monitoring or debugging purposes.
> +
> +	Example::
> +
> +	  echo 1 > memory.stat_refresh
> +	  cat memory.stat
> +
>    memory.numa_stat
>  	A read-only nested-keyed file which exists on non-root cgroups.
> 
> @@ -2173,7 +2190,7 @@ of the two is enforced.
> 
>  cgroup writeback requires explicit support from the underlying
>  filesystem.  Currently, cgroup writeback is implemented on ext2, ext4,
> -btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are
> +btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are
>  attributed to the root cgroup.

Same here, not sure what's changed...

>  There are inherent differences in memory and writeback management
> diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
> index 6358464bb416..a14d4d74c9aa 100644
> --- a/mm/memcontrol-v1.h
> +++ b/mm/memcontrol-v1.h
> @@ -4666,6 +4675,10 @@ static struct cftype memory_files[] = {
>  		.name = "stat",
>  		.seq_show = memory_stat_show,
>  	},
> +	{
> +		.name = "stat_refresh",
> +		.write = memory_stat_refresh_write,

I think we should use the CFTYPE_NOT_ON_ROOT flag to avoid creating
the file for the root cgroup if that's intended?

-- 
Cheers,
Harry / Hyeonggon

> +	},
>  #ifdef CONFIG_NUMA
>  	{
>  		.name = "numa_stat",
> --
> 2.51.2
> 
>