Re: [lkp-robot] [mm] 7674270022: will-it-scale.per_process_ops -19.3% regression

From: Nadav Amit <nadav.amit@gmail.com>
To: Minchan Kim <minchan@kernel.org>
Cc: linux-arch@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	Jeff Dike <jdike@addtoit.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	"David S. Miller" <davem@davemloft.net>,
	lkp@01.org, Russell King <linux@armlinux.org.uk>,
	Tony Luck <tony.luck@intel.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel test robot <xiaolong.ye@intel.com>
Subject: Re: [lkp-robot] [mm] 7674270022: will-it-scale.per_process_ops -19.3% regression
Date: Tue, 8 Aug 2017 01:16:08 -0700	[thread overview]
Message-ID: <CAKLkAJ48yH1hT2UQvxGf1i5Zuceasyu_4og3sqADuxOCD81c0Q@mail.gmail.com> (raw)
In-Reply-To: <20170808080821.GA31730@bbox>

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On Aug 8, 2017 01:08, "Minchan Kim" <minchan@kernel.org> wrote:

On Mon, Aug 07, 2017 at 10:51:00PM -0700, Nadav Amit wrote:
> Nadav Amit <nadav.amit@gmail.com> wrote:
>
> > Minchan Kim <minchan@kernel.org> wrote:
> >
> >> Hi,
> >>
> >> On Tue, Aug 08, 2017 at 09:19:23AM +0800, kernel test robot wrote:
> >>> Greeting,
> >>>
> >>> FYI, we noticed a -19.3% regression of will-it-scale.per_process_ops
due to commit:
> >>>
> >>>
> >>> commit: 76742700225cad9df49f05399381ac3f1ec3dc60 ("mm: fix
MADV_[FREE|DONTNEED] TLB flush miss problem")
> >>> url: https://github.com/0day-ci/linux/commits/Nadav-Amit/mm-
migrate-prevent-racy-access-to-tlb_flush_pending/20170802-205715
> >>>
> >>>
> >>> in testcase: will-it-scale
> >>> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
with 64G memory
> >>> with following parameters:
> >>>
> >>>   nr_task: 16
> >>>   mode: process
> >>>   test: brk1
> >>>   cpufreq_governor: performance
> >>>
> >>> test-description: Will It Scale takes a testcase and runs it from 1
through to n parallel copies to see if the testcase will scale. It builds
both a process and threads based test in order to see any differences
between the two.
> >>> test-url: https://github.com/antonblanchard/will-it-scale
> >>
> >> Thanks for the report.
> >> Could you explain what kinds of workload you are testing?
> >>
> >> Does it calls frequently madvise(MADV_DONTNEED) in parallel on multiple
> >> threads?
> >
> > According to the description it is "testcase:brk increase/decrease of
one
> > page”. According to the mode it spawns multiple processes, not threads.
> >
> > Since a single page is unmapped each time, and the iTLB-loads increase
> > dramatically, I would suspect that for some reason a full TLB flush is
> > caused during do_munmap().
> >
> > If I find some free time, I’ll try to profile the workload - but feel
free
> > to beat me to it.
>
> The root-cause appears to be that tlb_finish_mmu() does not call
> dec_tlb_flush_pending() - as it should. Any chance you can take care of
it?

Oops, but with second looking, it seems it's not my fault. ;-)
https://marc.info/?l=linux-mm&m=150156699114088&w=2

Err... Sorry for that...

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