linux-kernel - Re: [PATCH v12 00/31] Speculative page faults

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Date:   Sun, 13 Dec 2020 21:03:56 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Chinwen Chang <chinwen.chang@...iatek.com>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Haiyan Song <haiyanx.song@...el.com>,
        akpm@...ux-foundation.org, mhocko@...nel.org, peterz@...radead.org,
        kirill@...temov.name, ak@...ux.intel.com, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        aneesh.kumar@...ux.ibm.com, benh@...nel.crashing.org,
        mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        sergey.senozhatsky.work@...il.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        vinayak menon <vinayakm.list@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        zhong jiang <zhongjiang@...wei.com>,
        Balbir Singh <bsingharora@...il.com>, sj38.park@...il.com,
        Michel Lespinasse <walken@...gle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, npiggin@...il.com,
        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        miles.chen@...iatek.com
Subject: Re: [PATCH v12 00/31] Speculative page faults

On Tue, Jul 07, 2020 at 01:31:37PM +0800, Chinwen Chang wrote:
[..]
> > > Hi Laurent,
> > > 
> > > We merged SPF v11 and some patches from v12 into our platforms. After
> > > several experiments, we observed SPF has obvious improvements on the
> > > launch time of applications, especially for those high-TLP ones,
> > > 
> > > # launch time of applications(s):
> > > 
> > > package           version      w/ SPF      w/o SPF      improve(%)
> > > ------------------------------------------------------------------
> > > Baidu maps        10.13.3      0.887       0.98         9.49
> > > Taobao            8.4.0.35     1.227       1.293        5.10
> > > Meituan           9.12.401     1.107       1.543        28.26
> > > WeChat            7.0.3        2.353       2.68         12.20
> > > Honor of Kings    1.43.1.6     6.63        6.713        1.24
> > 
> > That's great news, thanks for reporting this!
> > 
> > > 
> > > By the way, we have verified our platforms with those patches and
> > > achieved the goal of mass production.
> > 
> > Another good news!
> > For my information, what is your targeted hardware?
> > 
> > Cheers,
> > Laurent.
> 
> Hi Laurent,
> 
> Our targeted hardware belongs to ARM64 multi-core series.

Hello!

I was trying to develop an intuition about why does SPF give improvement for
you on small CPU systems. This is just a high-level theory but:

1. Assume the improvement is because of elimination of "blocking" on
mmap_sem.
Could it be that the mmap_sem is acquired in write-mode unnecessarily in some
places, thus causing blocking on mmap_sem in other paths? If so, is it
feasible to convert such usages to acquiring them in read-mode?

2. Assume the improvement is because of lesser read-side contention on
mmap_sem.
On small CPU systems, I would not expect reducing cache-line bouncing to give
such a dramatic improvement in performance as you are seeing.

Thanks for any insight on this!

- Joel