[PATCH v2] mm/bpf: Always flush memcg stats when explicitly requested

[Runli <mingyu.he@shopee.com>]

memcg_rstat_updated() stops walking the ancestors when a memcg exceeds
the flush threshold. A concurrent flush can leave an ancestor below the
threshold, so the early exit prevents it from receiving further updates.

Consider root and leaf on a 4-CPU system, with a threshold of 256.
CPU 0 is flushing another CPU's stats after CPU 1's stats have already
been flushed. The counts below are shared stats_updates counters:

  CPU 0 (root flush)                CPU 1 (leaf updates)
  ------------------               --------------------
  clear leaf->stats_updates
                                   add 64 to leaf and root
  clear root->stats_updates
  finish flush
                                   leaf = 64, root = 0
                                   4 more batches of 64:
                                     leaf = 320, root = 256
                                   next update:
                                     leaf > threshold
                                     break; root is not updated

Root remains at the threshold, which is insufficient to trigger a flush,
while further leaf updates keep taking the early exit. The periodic
forced flush restores progress, but until then the root's aggregated
statistics can fall far behind. For example, page cache grows at rate of
1 GiB/s, a two-second wait can leave the reported usage about 2 GiB below
the actual usage.

Make bpf_mem_cgroup_flush_stats() bypass the update threshold so BPF
callers can explicitly flush stale statistics without waiting for the
periodic flush.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Link: https://lore.kernel.org/all/avKy9oMwDRjKjfzm@linux.dev/
Signed-off-by: Runli <mingyu.he@shopee.com>
---
Changes since v1:
- Make the BPF kfunc flush unconditionally, as suggested by Shakeel.
- Leave memcg_rstat_updated() unchanged.

v1: https://lore.kernel.org/all/20260911032905.63683-1-mingyu.he@shopee.com/

 mm/bpf_memcontrol.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/bpf_memcontrol.c b/mm/bpf_memcontrol.c
index d8f579c28560..f8552651b6d6 100644
--- a/mm/bpf_memcontrol.c
+++ b/mm/bpf_memcontrol.c
@@ -7,6 +7,7 @@
 
 #include <linux/memcontrol.h>
 #include <linux/bpf.h>
+#include <linux/cgroup.h>
 
 #include "internal.h"
 
@@ -154,11 +155,12 @@ __bpf_kfunc unsigned long bpf_mem_cgroup_page_state(struct mem_cgroup *memcg, in
  * bpf_mem_cgroup_flush_stats - Flush memory cgroup's statistics
  * @memcg: memory cgroup
  *
- * Propagate memory cgroup's statistics up the cgroup tree.
+ * Flush the memory cgroup subtree and propagate its statistics up the cgroup
+ * tree, regardless of the number of pending updates.
  */
 __bpf_kfunc void bpf_mem_cgroup_flush_stats(struct mem_cgroup *memcg)
 {
-	mem_cgroup_flush_stats(memcg);
+	css_rstat_flush(&memcg->css);
 }
 
 /**
-- 
2.39.2 (Apple Git-143)