Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail

From: Michal Hocko <mhocko@kernel.org>
To: Qian Cai <cai@lca.pw>
Cc: Feng Tang <feng.tang@intel.com>,
	kernel test robot <rong.a.chen@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Matthew Wilcox <willy@infradead.org>,
	Mel Gorman <mgorman@suse.de>, Kees Cook <keescook@chromium.org>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Iurii Zaikin <yzaikin@google.com>,
	andi.kleen@intel.com, tim.c.chen@intel.com,
	dave.hansen@intel.com, ying.huang@intel.com, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, lkp@lists.01.org
Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail
Date: Tue, 7 Jul 2020 15:56:43 +0200	[thread overview]
Message-ID: <20200707135643.GV5913@dhcp22.suse.cz> (raw)
In-Reply-To: <20200707130436.GA992@lca.pw>

On Tue 07-07-20 09:04:36, Qian Cai wrote:
> On Tue, Jul 07, 2020 at 02:06:19PM +0200, Michal Hocko wrote:
> > On Tue 07-07-20 07:43:48, Qian Cai wrote:
> > > 
> > > 
> > > > On Jul 7, 2020, at 6:28 AM, Michal Hocko <mhocko@kernel.org> wrote:
> > > > 
> > > > Would you have any examples? Because I find this highly unlikely.
> > > > OVERCOMMIT_NEVER only works when virtual memory is not largerly
> > > > overcommited wrt to real memory demand. And that tends to be more of
> > > > an exception rather than a rule. "Modern" userspace (whatever that
> > > > means) tends to be really hungry with virtual memory which is only used
> > > > very sparsely.
> > > > 
> > > > I would argue that either somebody is running an "OVERCOMMIT_NEVER"
> > > > friendly SW and this is a permanent setting or this is not used at all.
> > > > At least this is my experience.
> > > > 
> > > > So I strongly suspect that LTP test failure is not something we should
> > > > really lose sleep over. It would be nice to find a way to flush existing
> > > > batches but I would rather see a real workload that would suffer from
> > > > this imprecision.
> > > 
> > > I hear you many times that you really don’t care about those use
> > > cases unless you hear exactly people are using in your world.
> > > 
> > > For example, when you said LTP oom tests are totally artificial last
> > > time and how less you care about if they are failing, and I could only
> > > enjoy their efficiencies to find many issues like race conditions
> > > and bad error accumulation handling etc that your “real world use
> > > cases” are going to take ages or no way to flag them.
> > 
> > Yes, they are effective at hitting corner cases and that is fine. I
> > am not dismissing their usefulness. I have tried to explain that many
> > times but let me try again. Seeing a corner case and think about a
> > potential fix is one thing. On the other hand it is not really ideal to
> > treat such a failure a hard regression and consider otherwise useful
> 
> Well, terms like "corner cases" and "hard regression" are rather
> subjective.

Existing real life examples really makes them less subjective though.

[...]

> > LTP is a very useful tool to raise awareness of potential problems but
> > you shouldn't really follow those results just blindly.
> 
> You must think I am a newbie tester to give me this piece of advice
> then.

Not by even close. I can clearly see your involvement in testing and how
many good bug reports that results in.
-- 
Michal Hocko
SUSE Labs