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

[Dmitry Ilvokhin <d@ilvokhin.com>]

On Wed, Apr 01, 2026 at 07:46:35PM +0200, Vlastimil Babka (SUSE) wrote:

[...]

> > +/*
> > + * 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;
> 
> Hm doesn't this mean that lower id cpus will consistently fire in shorter
> intervals and higher id in longer intervals? What we want is same interval
> but differently offset, no?

Yes, I think that's a valid concern, this effectively skews the
interval rather than just introducing a phase offset.

I initially thought this might explain the increase in max wait, but it
turns out the columns were just swapped.

Spreading the initial scheduling and then requeueing with a constant
interval sounds like a reasonable alternative, e.g. below.

From 56ed7e17b32f0a7ce433caed87650b0de8246c4e Mon Sep 17 00:00:00 2001
From: Dmitry Ilvokhin <d@ilvokhin.com>
Date: Thu, 2 Apr 2026 04:49:06 -0700
Subject: [PATCH] mm/vmstat: stagger per-cpu vmstat updates to avoid zone->lock
 contention

Fix by spreading the shepherd's initial wakeup across the stat plain
sysctl_stat_interval to preserve the stagger. Every CPU still fires once
per interval, same frequency, different phase.

Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>
---
 mm/vmstat.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 2370c6fb1fcd..aee99786718a 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -2042,7 +2042,7 @@ 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));
+				sysctl_stat_interval);
 	}
 }
 
@@ -2140,7 +2140,8 @@ static void vmstat_shepherd(struct work_struct *w)
 				continue;
 
 			if (!delayed_work_pending(dw) && need_update(cpu))
-				queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
+				queue_delayed_work_on(cpu, mm_percpu_wq, dw,
+					(sysctl_stat_interval * cpu) / nr_cpu_ids);
 		}
 
 		cond_resched();
-- 
2.52.0