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

[Breno Leitao <leitao@debian.org>]

Hello Matthew,

On Thu, Apr 02, 2026 at 01:49:34PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 01, 2026 at 06:57:50AM -0700, 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.
> 
> But, uh, round_jiffies_relative() is _supposed_ to do that!  Look at
> both the documentation and implementation.  Why isn't it working?

The documentation is a bit trick on me,  sorry. This is what I read:

  "By rounding these timers to whole seconds, all such timers will fire
   at the same time, rather than at various times spread out. The goal
   of this is to have the CPU wake up less, which saves power."

This is the full documentation:

/**
 * round_jiffies_relative - function to round jiffies to a full second
 * @j: the time in (relative) jiffies that should be rounded
 *
 * round_jiffies_relative() rounds a time delta  in the future (in jiffies)
 * up or down to (approximately) full seconds. This is useful for timers
 * for which the exact time they fire does not matter too much, as long as
 * they fire approximately every X seconds.
 *
 * By rounding these timers to whole seconds, all such timers will fire
 * at the same time, rather than at various times spread out. The goal
 * of this is to have the CPU wake up less, which saves power.
 *
 * The return value is the rounded version of the @j parameter.
 */
unsigned long round_jiffies_relative(unsigned long j)
{
        return __round_jiffies_relative(j, raw_smp_processor_id());
}
EXPORT_SYMBOL_GPL(round_jiffies_relative);