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Date:   Tue, 7 Jul 2020 12:28:35 +0200
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
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On Sun 05-07-20 11:52:32, Qian Cai wrote:
> On Sun, Jul 05, 2020 at 08:58:54PM +0800, Feng Tang wrote:
> > On Sun, Jul 05, 2020 at 08:15:03AM -0400, Qian Cai wrote:
> > > 
> > > 
> > > > On Jul 5, 2020, at 12:45 AM, Feng Tang <feng.tang@intel.com> wrote:
> > > > 
> > > > I did reproduce the problem, and from the debugging, this should
> > > > be the same root cause as lore.kernel.org/lkml/20200526181459.GD991@lca.pw/
> > > > that loosing the batch cause some accuracy problem, and the solution of
> > > > adding some sync is still needed, which is dicussed in
> > > 
> > > Well, before taking any of those patches now to fix the regression,
> > > we will need some performance data first. If it turned out the
> > > original performance gain is no longer relevant anymore due to this
> > > regression fix on top, it is best to drop this patchset and restore
> > > that VM_WARN_ONCE, so you can retry later once you found a better
> > > way to optimize.
> > 
> > The fix of adding sync only happens when the memory policy is being
> > changed to OVERCOMMIT_NEVER, which is not a frequent operation in
> > normal cases.
> > 
> > For the performance improvment data both in commit log and 0day report
> > https://lore.kernel.org/lkml/20200622132548.GS5535@shao2-debian/
> > it is for the will-it-scale's mmap testcase, which will not runtime
> > change memory overcommit policy, so the data should be still valid
> > with this fix.
> 
> Well, I would expect people are perfectly reasonable to use
> OVERCOMMIT_NEVER for some workloads making it more frequent operations.

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.

On the other hand perf. boost with larger batches with defualt overcommit
setting sounds like a nice improvement to have.
-- 
Michal Hocko
SUSE Labs