[PATCH] mm/vmstat: spread vmstat_update requeue across the stat interval

[Breno Leitao <leitao@debian.org>]

vmstat_update uses round_jiffies_relative() when re-queuing itself,
which aligns all CPUs' timers to the same second boundary.  When many
CPUs have pending PCP pages to drain, they all call decay_pcp_high() ->
free_pcppages_bulk() simultaneously, serializing on zone->lock and
hitting contention.

Introduce vmstat_spread_delay() which distributes each CPU's
vmstat_update evenly across the stat interval instead of aligning them.

This does not increase the number of timer interrupts — each CPU still
fires once per interval. The timers are simply staggered rather than
aligned. Additionally, vmstat_work is DEFERRABLE_WORK, so it does not
wake idle CPUs regardless of scheduling; the spread only affects CPUs
that are already active

`perf lock contention` shows 7.5x reduction in zone->lock contention
(872 -> 117 contentions, 199ms -> 81ms total wait) on a 72-CPU aarch64
system under memory pressure.

Tested on a 72-CPU aarch64 system using stress-ng --vm to generate
memory allocation bursts.  Lock contention was measured with:

  perf lock contention -a -b -S free_pcppages_bulk

Results with KASAN enabled:

  free_pcppages_bulk contention (KASAN):
  +--------------+----------+----------+
  | Metric       | No fix   | With fix |
  +--------------+----------+----------+
  | Contentions  |      872 |      117 |
  | Total wait   | 199.43ms | 80.76ms  |
  | Max wait     |   4.19ms | 35.76ms  |
  +--------------+----------+----------+

Results without KASAN:

  free_pcppages_bulk contention (no KASAN):
  +--------------+----------+----------+
  | Metric       | No fix   | With fix |
  +--------------+----------+----------+
  | Contentions  |      240 |      133 |
  | Total wait   |  34.01ms | 24.61ms  |
  | Max wait     |   965us  |  1.35ms  |
  +--------------+----------+----------+

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/vmstat.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 2370c6fb1fcd..2e94bd765606 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -2032,6 +2032,29 @@ static int vmstat_refresh(const struct ctl_table *table, int write,
 }
 #endif /* CONFIG_PROC_FS */
 
+/*
+ * Return a per-cpu delay that spreads vmstat_update work across the stat
+ * interval.  Without this, round_jiffies_relative() aligns every CPU's
+ * timer to the same second boundary, causing a thundering-herd on
+ * zone->lock when multiple CPUs drain PCP pages simultaneously via
+ * decay_pcp_high() -> free_pcppages_bulk().
+ */
+static unsigned long vmstat_spread_delay(void)
+{
+	unsigned long interval = sysctl_stat_interval;
+	unsigned int nr_cpus = num_online_cpus();
+
+	if (nr_cpus <= 1)
+		return round_jiffies_relative(interval);
+
+	/*
+	 * Spread per-cpu vmstat work evenly across the interval.  Don't
+	 * use round_jiffies_relative() here -- it would snap every CPU
+	 * back to the same second boundary, defeating the spread.
+	 */
+	return interval + (interval * (smp_processor_id() % nr_cpus)) / nr_cpus;
+}
+
 static void vmstat_update(struct work_struct *w)
 {
 	if (refresh_cpu_vm_stats(true)) {
@@ -2042,7 +2065,7 @@ static void vmstat_update(struct work_struct *w)
 		 */
 		queue_delayed_work_on(smp_processor_id(), mm_percpu_wq,
 				this_cpu_ptr(&vmstat_work),
-				round_jiffies_relative(sysctl_stat_interval));
+				vmstat_spread_delay());
 	}
 }
 

---
base-commit: cf7c3c02fdd0dfccf4d6611714273dcb538af2cb
change-id: 20260401-vmstat-048e0feaf344

Best regards,
--  
Breno Leitao <leitao@debian.org>