Re: [RFC PATCH v2] Introduce Hierarchical Per-CPU Counters

[Paul E. McKenney]

On Tue, Apr 08, 2025 at 01:44:25PM -0700, Christoph Lameter (Ampere) wrote:
> On Tue, 8 Apr 2025, Mathieu Desnoyers wrote:
> 
> > Currently percpu_counter_tree_precise_sum_unbiased() iterates on each
> > possible cpu, which does not require cpu hotplug integration.
> 
> Well that looks like a performance issue if you have a system that can
> expand to 8K cpus but currently only has 16 or so online.

RCU handles this by iterating from zero to nr_cpu_ids, which is set during
early boot.  It also builds its tree-shaped data structures during early
boot based on nr_cpu_ids.

							Thanx, Paul