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

[Usama Arif <usama.arif@linux.dev>]

On 01/04/2026 18:43, Breno Leitao wrote:
> On Wed, Apr 01, 2026 at 08:23:40AM -0700, Usama Arif wrote:
>> On Wed, 01 Apr 2026 06:57:50 -0700 Breno Leitao <leitao@debian.org> wrote:
>>
>>> 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());
>>
>> This is awesome! Maybe this needs to be done to vmstat_shepherd() as well?
>>
>> vmstat_shepherd() still queues work with delay 0 on all CPUs that
>> need_update() in its for_each_online_cpu() loop:
>>
>>       if (!delayed_work_pending(dw) && need_update(cpu))
>>           queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
>>
>> So when the shepherd fires, it kicks all dormant CPUs' vmstat workers
>> simultaneously.
>>
>> Under sustained memory pressure on a large system, I think the shepherd
>> fires every sysctl_stat_interval and could re-trigger the same lock
>> contention?
> 
> Good point - incorporating similar spreading logic in vmstat_shepherd()
> would indeed address the simultaneous queueing issue you've described.
> 
> Should I include this in a v2 of this patch, or would you prefer it as
> a separate follow-up patch?

I think it can be a separate follow-up patch, but no strong preference.
For this patch:

Acked-by: Usama Arif <usama.arif@linux.dev>