linux-kernel - Re: [PATCH] x86: unify x86 Makefile(s)

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Date:	Sat, 29 Dec 2007 19:58:42 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Andi Kleen <ak@...e.de>
Cc:	Adrian Bunk <bunk@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: unify x86 Makefile(s)

On Sat, Dec 29, 2007 at 07:24:51PM +0100, Andi Kleen wrote:
> 
> > 
> > This is the version that should exhibit the behaviour with increased text size.
> 
> The issue was not text size (u-a-a-t generally decreases that) but maximum stack usage.
> You need to check make checkstack output.

gcc 3.4.5 with: -fno-unit-at-a-time:
0xc0173b9a do_sys_poll [vmlinux]:			852
0xc0173cc2 do_sys_poll [vmlinux]:			852
0xc0206f8e nlmclnt_reclaim [vmlinux]:			704
0xc020701e nlmclnt_reclaim [vmlinux]:			704
0xc01a2c83 reiserfs_rename [vmlinux]:			668
0xc01a34a0 reiserfs_rename [vmlinux]:			668
0xc0173052 do_select [vmlinux]:				656
0xc0173320 do_select [vmlinux]:				656
0xc0417ceb do_ipv6_setsockopt [vmlinux]:		608
0xc0418746 do_ipv6_setsockopt [vmlinux]:		608

gcc 3.4.5 with: -funit-at-a-time:
0xc02926e8 lo_ioctl [vmlinux]:				1336
0xc0293055 lo_ioctl [vmlinux]:				1336
0xc03bf222 dev_ethtool [vmlinux]:			908
0xc03c00e5 dev_ethtool [vmlinux]:			908
0xc036adf9 cdrom_open [vmlinux]:			892
0xc036b78d cdrom_open [vmlinux]:			892
0xc0172445 do_sys_poll [vmlinux]:			860
0xc01726e9 do_sys_poll [vmlinux]:			860
0xc020fca3 sys_semctl [vmlinux]:			796
0xc0210447 sys_semctl [vmlinux]:			796
0xc02058af nlmclnt_reclaim [vmlinux]:			704
0xc020593f nlmclnt_reclaim [vmlinux]:			704
0xc01719d4 do_select [vmlinux]:				668
0xc0171d59 do_select [vmlinux]:				668
0xc01a121a reiserfs_rename [vmlinux]:			668
0xc01a1a54 reiserfs_rename [vmlinux]:			668
0xc0412e39 ipv6_setsockopt [vmlinux]:			612
0xc04138ca ipv6_setsockopt [vmlinux]:			612

So here we indeed see the expected (bad) behaviour.
unit-at-a-time causes much higher stack usage in some spots.
Above is listed only the hits above 600 bytes.

Until we are on par with stack usage I recommend to keep
-fno-unit-at-a-time disabled for gcc less than 4.00 as
suggested by Adrian (as is what we have today).

	Sam
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