Re: [PATCH 1/2] mm: NUMA stats code cleanup and enhancement

[Andi Kleen]

Vlastimil Babka <vbabka@xxxxxxx> writes:
>
> I'm worried about the "for_each_possible..." approach here and elsewhere
> in the patch as it can be rather excessive compared to the online number
> of cpus (we've seen BIOSes report large numbers of possible CPU's). IIRC

Even if they report a few hundred extra reading some more shared cache lines
is very cheap. The prefetcher usually quickly figures out such a pattern
and reads it all in parallel.

I doubt it will be noticeable, especially not in a slow path
like reading something from proc/sys.

> the general approach with vmstat is to query just online cpu's / nodes,
> and if they go offline, transfer their accumulated stats to some other
> "victim"?

That's very complicated, and unlikely to be worth it.

-Andi