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Date:   Sun, 5 Jul 2020 22:36:14 -0400
From:   Qian Cai <cai@lca.pw>
To:     Feng Tang <feng.tang@intel.com>
Cc:     kernel test robot <rong.a.chen@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        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 Mon, Jul 06, 2020 at 09:43:13AM +0800, Feng Tang wrote:
> On Sun, Jul 05, 2020 at 11:52:32AM -0400, 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.
> 
> In my last email, I was not saying OVERCOMMIT_NEVER is not a normal case,
> but I don't think user will too frequently runtime change the overcommit
> policy. And the fix patch of syncing 'vm_committed_as' is only called when
> user calls 'sysctl -w vm.overcommit_memory=2'.
> 
> > The question is now if any of those regression fixes would now regress
> > performance of OVERCOMMIT_NEVER workloads or just in-par with the data
> > before the patchset?
> 
> For the original patchset, it keeps vm_committed_as unchanged for
> OVERCOMMIT_NEVER policy and enlarge it for the other 2 loose policies
> OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS, and I don't expect the "OVERCOMMIT_NEVER
> workloads" performance  will be impacted. If you have suggetions for this
> kind of benchmarks, I can test them to better verify the patchset, thanks!

Then, please capture those information into a proper commit log when you
submit the regression fix on top of the patchset, and CC PER-CPU MEMORY
ALLOCATOR maintainers, so they might be able to review it properly.

> 
> - Feng
> 
> > 
> > Given now this patchset has had so much churn recently, I would think
> > "should be still valid" is not really the answer we are looking for.
> > 
> > > 
> > > Thanks,
> > > Feng
> > > 
> > >