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

[kemi]

On 2017å09æ15æ 22:28, Michal Hocko wrote:
> 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.
> 

It has benefit, as Dave mentioned above.
Actually, it adds some coding complexity to provide a tuning interface with
on/off/auto mode. Using human-readable string instead of magic number makes
it easier to use, people probably don't need to review the ABI doc again
before using it. So, I don't think that should be a problem 
 
>>> 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
> 

If it really matters, setting vmstat_mode=strict as a default option is a simple 
way to fix it. What's your idea? thanks

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