Date:   Mon, 18 Sep 2017 07:50:06 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     kemi <kemi.wang@...el.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Johannes Weiner <hannes@...xchg.org>,
        Christopher Lameter <cl@...ux.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Ying Huang <ying.huang@...el.com>,
        Aaron Lu <aaron.lu@...el.com>,
        Proc sysctl <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] mm, sysctl: make VM stats configurable

On Mon 18-09-17 10:44:52, kemi wrote:
> 
> 
> On 2017年09月15日 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 this a thing that would be changed very often. I suspect that once
needed it will be set in a startup sysctl configuration and there will
be no further need to touch it again.

> >>> 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

Well, we are usually very conservative when changing the default
behavior. The primary reason why I was asking is that the auto mode
doesn't make much sense unless it is the default. I fully realize that
such an hypothetical breakage is really hard to envision but considering
it is more code to allow auto mode than a simple on/off (we have parsing
helpers for that AFAIR) then I would rather go with the simpler option.

This is up to you of course.
-- 
Michal Hocko
SUSE Labs

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