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

[Kiryl Shutsemau <kas@kernel.org>]

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.

Nice idea.

> 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.

Wow. That's huge improvement.

> 
> 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>

Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov