linux-kernel - Re: [PATCH v2] mm/page_alloc.c: inline _

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Date:   Wed, 11 Oct 2017 10:34:02 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Dave Hansen <dave.hansen@...el.com>, linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Huang Ying <ying.huang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] mm/page_alloc.c: inline __rmqueue()

On Tue, Oct 10, 2017 at 02:45:45PM -0700, Andrew Morton wrote:
> On Tue, 10 Oct 2017 13:43:43 +0800 Aaron Lu <aaron.lu@...el.com> wrote:
> 
> > On Mon, Oct 09, 2017 at 10:19:52PM -0700, Dave Hansen wrote:
> > > On 10/09/2017 07:56 PM, Aaron Lu wrote:
> > > > This patch adds inline to __rmqueue() and vmlinux' size doesn't have any
> > > > change after this patch according to size(1).
> > > > 
> > > > without this patch:
> > > >    text    data     bss     dec     hex     filename
> > > > 9968576 5793372 17715200  33477148  1fed21c vmlinux
> > > > 
> > > > with this patch:
> > > >    text    data     bss     dec     hex     filename
> > > > 9968576 5793372 17715200  33477148  1fed21c vmlinux
> > > 
> > > This is unexpected.  Could you double-check this, please?
> > 
> > mm/page_alloc.o has size changes:
> > 
> > Without this patch:
> > $ size mm/page_alloc.o
> >   text    data     bss     dec     hex filename
> >  36695    9792    8396   54883    d663 mm/page_alloc.o
> > 
> > With this patch:
> > $ size mm/page_alloc.o
> >   text    data     bss     dec     hex filename
> >  37511    9792    8396   55699    d993 mm/page_alloc.o
> > 
> > But vmlinux doesn't.
> > 
> > It's not clear to me what happened, do you want to me dig this out?
> 
> There's weird stuff going on.
> 
> With x86_64 gcc-4.8.4
> 
> Patch not applied:
> 
> akpm3:/usr/local/google/home/akpm/k/25> nm mm/page_alloc.o|grep __rmqueue
> 0000000000002a00 t __rmqueue
> 
> Patch applied:
> 
> akpm3:/usr/local/google/home/akpm/k/25> nm mm/page_alloc.o|grep __rmqueue
> 000000000000039f t __rmqueue_fallback
> 0000000000001220 t __rmqueue_smallest
> 
> So inlining __rmqueue has caused the compiler to decide to uninline
> __rmqueue_fallback and __rmqueue_smallest, which largely undoes the
> effect of your patch.
> 
> `inline' is basically advisory (or ignored) in modern gcc's.  So gcc
> has felt free to ignore it in __rmqueue_fallback and __rmqueue_smallest
> because gcc thinks it knows best.  That's why we created
> __always_inline, to grab gcc by the scruff of its neck.

This is a good point and I agree with Andi to use always_inline for
those functions that we really want to inline.

> 
> So...  I think this patch could do with quite a bit more care, tuning
> and testing with various gcc versions.

I did some more testing.

With x86_64 gcc-4.6.3 available from kernel.org crosstool:

Patch not applied:

[aaron@...onlu linux]$ nm mm/page_alloc.o |grep __rmqueue
00000000000023f0 t __rmqueue
00000000000027c0 t __rmqueue_pcplist.isra.95

Patch applied:

[aaron@...onlu linux]$ nm mm/page_alloc.o |grep __rmqueue
0000000000002950 t __rmqueue_pcplist.isra.95

Works expected.

With self built x86_64 gcc-4.8.4:

Patch not applied:

[aaron@...onlu linux]$ nm mm/page_alloc.o |grep __rmqueue
0000000000001f20 t __rmqueue

Patch applied:

[aaron@...onlu linux]$ nm mm/page_alloc.o |grep __rmqueue

Works expected.(conflicts with your result though).

I also tested gcc-4.9.4, gcc-5.3.1, gcc-6.4.0 and gcc-7.2.1, all have
the same output as the above gcc-4.8.4.

Then I realized CONFIG_OPTIMIZE_INLINING which I always disabled as
suggested by the help message(If unsure, say N). Turnining that config
on indeed caused gcc-4.8.4 to emit __rmqueue_fallback here.

I think I'll just mark those functions always_inline.