[PATCH v2 8/8] mm/vmstat: Avoid redundant per-CPU stat_threshold writes in refresh_zone_stat_thresholds()

[sashiko-bot at kernel.org]

> During CPU hotplug, vmstat_cpu_online() and vmstat_cpu_dead() invoke
> refresh_zone_stat_thresholds() on every CPU transition. Currently,
> refresh_zone_stat_thresholds() first zeroes
> per_cpu_nodestats->stat_threshold across all online CPUs and all online
> nodes, and then unconditionally writes per_cpu_zonestats->stat_threshold
> and per_cpu_nodestats->stat_threshold inside the zone loop for every
> populated zone and every online CPU.
> 
> However, calculate_normal_threshold(zone) only changes when
> fls(num_online_cpus()) changes (at powers of 2) and is clamped at 125
> for large normal zones (>= 16 GB) once num_online_cpus() >= 64.
> Unconditionally zeroing and rewriting every CPU's per-CPU stat_threshold
> on every hotplug step repeatedly dirties remote per-CPU cachelines with
> identical values (and briefly exposes stat_threshold == 0 on live CPUs).
> 
> [ ... ]
>   Total:     3.46 s ->  3.38 s (-2.3%)
>   refresh_zone_stat_thresholds() (cpu511 offline / online):
>     104.6 us -> 50.7 us (-51.6%) / 109.3 us -> 53.5 us (-51.1%)
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261019011446.834487-1-pasha.tatashin@soleen.com?part=8