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

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Date:   Fri, 15 Sep 2017 16:28:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Kemi Wang <kemi.wang@...el.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 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