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

[Michal Hocko]

On Fri 15-09-17 07:16:23, Dave Hansen wrote:
> 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.

Just to make it clear, I am not really opposing. It just adds some code
which we can safe... It is also rather chatty for something that can be
true/false.
 
> > 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.

This is how I use them as well.
 
> 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.

yes

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

-- 
Michal Hocko
SUSE Labs