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: kernel test robot <xiaolong.ye@intel.com>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Russell King <linux@armlinux.org.uk>,
	Tony Luck <tony.luck@intel.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	"David S. Miller" <davem@davemloft.net>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Jeff Dike <jdike@addtoit.com>,
	linux-arch@vger.kernel.org, lkp@01.org
Subject: Re: [lkp-robot] [mm]  7674270022:  will-it-scale.per_process_ops -19.3% regression
Date: Mon, 7 Aug 2017 21:23:34 -0700	[thread overview]
Message-ID: <93CA4B47-95C2-43A2-8E92-B142CAB1DAF7@gmail.com> (raw)
In-Reply-To: <20170808022830.GA28570@bbox>

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.

Nadav 

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