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

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 4/1/26 7:46 PM, Vlastimil Babka (SUSE) wrote:
> On 4/1/26 15:57, Breno Leitao 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>
> 
> Cool!
> 
> I noticed __round_jiffies_relative() exists and the description looks like
> it's meant for exactly this use case?

On closer look, using round_jiffies_relative() as before your patch
means it's calling __round_jiffies_relative(j, raw_smp_processor_id())
so that's already doing this spread internally. You're also relying
smp_processor_id() so it's not about using a different cpu id.

But your patch has better results, why? I still think it's not doing
what it intends - I think it makes every cpu have different interval
length (up to twice the original length), not skew. Is it that, or that
the 3 jiffies skew per cpu used in round_jiffies_common() is
insufficient? Or it a bug in its skew implementation?

Ideally once that's clear, the findings could be used to improve
round_jiffies_common() and hopefully there's nothing here that's vmstat
specific.

Thanks,
Vlastimil

>> ---
>>  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;
> 
> 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?
> 
>> +}
>> +
>>  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>
>>
>