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Message-ID: <aRHMWewcW41Vdzed@tiehlicka>
Date: Mon, 10 Nov 2025 12:28:25 +0100
From: Michal Hocko <mhocko@...e.com>
To: Leon Huang Fu <leon.huangfu@...pee.com>
Cc: linux-mm@...ck.org, tj@...nel.org, mkoutny@...e.com, hannes@...xchg.org,
roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
muchun.song@...ux.dev, akpm@...ux-foundation.org,
joel.granados@...nel.org, jack@...e.cz, laoar.shao@...il.com,
mclapinski@...gle.com, kyle.meyer@....com, corbet@....net,
lance.yang@...ux.dev, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH mm-new v3] mm/memcontrol: Add memory.stat_refresh for
on-demand stats flushing
On Mon 10-11-25 18:19:48, 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 that is thousands of updates out of date.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@...pee.com>
Acked-by: Michal Hocko <mhocko@...e.com>
Thanks!
> ---
> 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(-)
>
> 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
> 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.
> +
> + 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.
>
> There are inherent differences in memory and writeback management
> diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
> index 6eed14bff742..c3eac9b1f1be 100644
> --- a/mm/memcontrol-v1.c
> +++ b/mm/memcontrol-v1.c
> @@ -2041,6 +2041,10 @@ struct cftype mem_cgroup_legacy_files[] = {
> .name = "stat",
> .seq_show = memory_stat_show,
> },
> + {
> + .name = "stat_refresh",
> + .write = memory_stat_refresh_write,
> + },
> {
> .name = "force_empty",
> .write = mem_cgroup_force_empty_write,
> 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
> @@ -29,6 +29,8 @@ void drain_all_stock(struct mem_cgroup *root_memcg);
> unsigned long memcg_events(struct mem_cgroup *memcg, int event);
> unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item);
> int memory_stat_show(struct seq_file *m, void *v);
> +ssize_t memory_stat_refresh_write(struct kernfs_open_file *of, char *buf,
> + size_t nbytes, loff_t off);
>
> void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n);
> struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg);
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index bfc986da3289..19ef4b971d8d 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -610,6 +610,15 @@ static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force)
> css_rstat_flush(&memcg->css);
> }
>
> +static void memcg_flush_stats(struct mem_cgroup *memcg, bool force)
> +{
> + if (mem_cgroup_disabled())
> + return;
> +
> + memcg = memcg ?: root_mem_cgroup;
> + __mem_cgroup_flush_stats(memcg, force);
> +}
> +
> /*
> * mem_cgroup_flush_stats - flush the stats of a memory cgroup subtree
> * @memcg: root of the subtree to flush
> @@ -621,13 +630,7 @@ static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force)
> */
> void mem_cgroup_flush_stats(struct mem_cgroup *memcg)
> {
> - if (mem_cgroup_disabled())
> - return;
> -
> - if (!memcg)
> - memcg = root_mem_cgroup;
> -
> - __mem_cgroup_flush_stats(memcg, false);
> + memcg_flush_stats(memcg, false);
> }
>
> void mem_cgroup_flush_stats_ratelimited(struct mem_cgroup *memcg)
> @@ -4530,6 +4533,12 @@ int memory_stat_show(struct seq_file *m, void *v)
> return 0;
> }
>
> +ssize_t memory_stat_refresh_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off)
> +{
> + memcg_flush_stats(mem_cgroup_from_css(of_css(of)), true);
> + return nbytes;
> +}
> +
> #ifdef CONFIG_NUMA
> static inline unsigned long lruvec_page_state_output(struct lruvec *lruvec,
> int item)
> @@ -4666,6 +4675,10 @@ static struct cftype memory_files[] = {
> .name = "stat",
> .seq_show = memory_stat_show,
> },
> + {
> + .name = "stat_refresh",
> + .write = memory_stat_refresh_write,
> + },
> #ifdef CONFIG_NUMA
> {
> .name = "numa_stat",
> --
> 2.51.2
--
Michal Hocko
SUSE Labs
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