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

Kiryl Shutsemau
2026-04-01T15:01:59Z

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