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

From: kemi
Date: Sun Sep 17 2017 - 22:46:16 EST

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.