Date: Fri, 15 Sep 2017 16:28:23 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Kemi Wang <kemi.wang@intel.com>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mel Gorman <mgorman@techsingularity.net>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Christopher Lameter <cl@linux.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Vlastimil Babka <vbabka@suse.cz>,
        Hillf Danton <hillf.zj@alibaba-inc.com>,
        Tim Chen <tim.c.chen@intel.com>, Andi Kleen <andi.kleen@intel.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Ying Huang <ying.huang@intel.com>, Aaron Lu <aaron.lu@intel.com>,
        Proc sysctl <linux-fsdevel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] mm, sysctl: make VM stats configurable
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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