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

[Breno Leitao <leitao@debian.org>]

Hello Johannes,

On Wed, Apr 01, 2026 at 10:25:35AM -0400, Johannes Weiner wrote:
> On Wed, Apr 01, 2026 at 06:57:50AM -0700, Breno Leitao wrote:
> > +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;
> 
> smp_processor_id() <= nr_cpus, so
> 
> 	return interval + interval*cpu/nr_cpus
> 
> should be equivalent, no?

nr_cpus is the number of online CPUs, while smp_processor_id() is the
CPU id.

If you offline a CPU, then smp_processor_id() might be bigger than
num_online_cpus()

My goal was to linearly shift the timer and avoid creating gaps when
removing certain CPUs.

Thanks for the review,
--breno