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

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Message-ID: <5553273B.6020405@redhat.com>
Date:	Wed, 13 May 2015 12:28:11 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
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

On 05/13/2015 12:17 PM, Ingo Molnar wrote:
>>> 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.

I think O2 difference is just noise: with -O2 gcc is far less prone
to bogus deinlining, my patch should have negligible effect.
And effect is indeed negligible: +70 bytes on 12 megabytes.


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