Re: [PATCH 1/3] mm, sysctl: make VM stats configurable

[Dave Hansen]

On 09/15/2017 04:49 AM, Michal Hocko wrote:
> Why do we need an auto-mode? Is it safe to enforce by default.

Do we *need* it?  Not really.

But, it does offer the best of both worlds: The vast majority of users
see virtually no impact from the counters.  The minority that do need
them pay the cost *and* don't have to change their tooling at all.

> Is it> possible that userspace can get confused to see 0 NUMA stats in
the
> first read while other allocation stats are non-zero?

I doubt it.  Those counters are pretty worthless by themselves.  I have
tooling that goes and reads them, but it aways displays deltas.  Read
stats, sleep one second, read again, print the difference.

The only scenario I can see mattering is someone who is seeing a
performance issue due to NUMA allocation misses (or whatever) and wants
to go look *back* in the past.

A single-time printk could also go a long way to keeping folks from
getting confused.