linux-kernel - Re: [PATCH] force inlining of spinlock ops

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Date:	Wed, 13 May 2015 12:17:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Graf <tgraf@...g.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Bart Van Assche <bvanassche@....org>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] force inlining of spinlock ops


* Denys Vlasenko <dvlasenk@...hat.com> wrote:

> On 05/12/2015 01:43 PM, Ingo Molnar wrote:
> >>> Furtermore, what is the size win on x86 defconfig with these options 
> >>> set?
> >>
> >> CONFIG_OPTIMIZE_INLINING=y is in defconfig.
> >>
> >> Size difference for CC_OPTIMIZE_FOR_SIZE:
> >>
> >>     text    data     bss      dec    hex filename
> >> 12335864 1746152 1081344 15163360 e75fe0 vmlinux.CC_OPTIMIZE_FOR_SIZE=y
> >> 10373764 1684200 1077248 13135212 c86d6c vmlinux.CC_OPTIMIZE_FOR_SIZE=n
> >>
> >> Decrease by about 19%.
> > 
> > I suspect the 'filename' field wants to be flipped?
> 
> Yes.
> 
> > In any case, the interesting measurement would not be -Os comparisons 
> > (which causes GCC to be too crazy), but to see the size effect of your 
> > _patch_ that always-inlines spinlock ops, on plain defconfig and on 
> > defconfig-Os.
> 
> Here it is:
> 
>     text    data     bss      dec    hex filename
> 12335864 1746152 1081344 15163360 e75fe0 vmlinuxO2.before
> 12335930 1746152 1081344 15163426 e76022 vmlinux

Hm, that's a (small) size increase on O2.

That might be a net positive though: because now we've eliminated 
quite a few function calls. Do we know which individual functions 
bloat and which debloat?

>     text    data     bss      dec    hex filename
> 10373764 1684200 1077248 13135212 c86d6c vmlinuxOs.before
> 10363621 1684200 1077248 13125069 c845cd vmlinux

A decrease - which gets exploded on allyesconfig.

So as long as the -O2 case does not get hurt we can do -Os fixes.

I think this needs a bit more work to ensure that the O2 case is a net 
win.

Thanks,

	Ingo
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