[FAQF] KC14: Kernel compiling, CFLAGS and make.conf settings

rac

Thanks, int1.

If the Gentoo developers wanted to make the make.conf CFLAGS apply to the kernel as well, they could rather easily (lostlogic patches the kernel fairly heavily as it is

). So I don't think it's really the distribution indpendence that's the reason the kernel CFLAGS are kept separate. I think it's simply the fact that the kernel has its own set of rather demanding restrictions and opinions about which flags it should be compiled with, that are independent of the choices made at the application level. I could certainly be wrong on this point, however.

It's also not necessary to edit Makefiles at all to change kernel CFLAGS, although that is one way to do it. Another way is to define the CFLAGS_KERNEL envariable when you compile.
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int1 · Tux's lil' helper Joined: 08 Nov 2002 Posts: 139

Didn't know that one. Thanks for the tip!

int1

PS> I can rewrite this with your tip if you'd like.

rac · Posted: Mon Dec 02, 2002 11:59 pm Post subject:

zypher · Posted: Fri Dec 06, 2002 6:56 am Post subject:

BTW: My kernel compilation warns about -malign-functions is obsolete and you should use -falign-functions (on 2.3.18-wolk3.8, which really should be in portage one day...).
Does this mean the function is translated to -falign-functions or is it discarded?
I'll change the Makefile and try it again anyway, just curious.
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ebrostig · Posted: Sat Dec 07, 2002 1:30 am Post subject:

rac · Posted: Sat Dec 07, 2002 7:14 am Post subject:

I think it's a typo of 2.4.18. -malign-functions changed names to -falign-functions somewhere between GCC 2.95 and 3.2.
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Zu` · l33t Joined: 26 May 2002 Posts: 716 Location: BE

This is interesting, but I wonder if it's worth it ? Especially since the configuration of your kernel (.config) will specify the arch. (-march=athlon in my case, while I compile everthing else with -march=athlon-xp.)

So what's useable? -O3 ? -fomit-frame-pointer ?

Are there any real advantages of compiling the kernel with modified CFLAGS? I.e. noticeable speed increase, or other .. ?

For the ones who have tested this, is it worth bothering with ?

Thanks.

rac · Posted: Mon Dec 09, 2002 9:25 pm Post subject:

Oh, yeah. Thanks, Zu'. Thanks for reminding me. There should probably also be something in here about the GCC3 patch in recent gentoo-sources, which adds "gcc 3+" options to the processor types config option. That allows another way to change -march. And, just to make sure things are clear, Zu', the point of this FAQ is not to encourage people to mess around with kernel CFLAGS, it's just that there have been several people that got confused by the fact that the kernel is not compiled with the CFLAGS in make.conf. They assumed that the kernel is just like any other software being compiled by portage, but it's not. This confusion has made some kernel compilation threads get confusing, when people start asking about and changing CFLAGS when it doesn't have any effect. That's why this FAQ entry was born.
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Zu` · l33t Joined: 26 May 2002 Posts: 716 Location: BE

Thanks for clarifying rac.

I'm using vanilla-sources and I wasn't aware of that patch in the gentoo-sources. I'll have a look at it. I wonder if it just sets the -march or 'forces' even more optimalisation.

About the FAQ, yes I see how it's meant to be used. Still, It'd be interesting to hear from people who have modified their kernel CFLAGS. Nevertheless, this isn't necessarily the right place to discuss this, we better keep FAQ's somewhat clean

Regards,

zu

rac · Posted: Mon Dec 09, 2002 10:49 pm Post subject:

int1 · Tux's lil' helper Joined: 08 Nov 2002 Posts: 139

int1 · Tux's lil' helper Joined: 08 Nov 2002 Posts: 139

Noticed this still isn't linked in the FAQ. Any specific reason?

int1

rac · Posted: Sat Jan 25, 2003 6:56 pm Post subject:

cazze · Posted: Wed Mar 26, 2003 10:33 pm Post subject:

Hi,

i tried to add -march=i586 for my c3 ezra processor in the Makefile, but with the compile, i got -march=i586 (the one i added) AND -march=i486.

i think it's important to say that if you change something like -march in your Makefile, you have to edit your .config file and deselect all the possible processors.

you can see wich flags are added for your processor in arch/i386(in my case)/Makefile

Hope this will help

kammicazze

bushwacker · n00b Joined: 22 Feb 2004 Posts: 37 Location: Tacoma, WA

is this information safe to use with the 2.6.x kernel line? I'd like to optimize my kernel a bit, but I dont want to do anything crazy until I know if/what settings can be applied. I have an Athlon XP CPU, so I"d like to use (if possible):

wdreinhart · Guru Joined: 11 Jun 2003 Posts: 569 Location: 4QFJ12345678

-mmmx -msse and -m3dnow are implied by -march=athlon-xp. You don't really need them anywhere.

bushwacker · n00b Joined: 22 Feb 2004 Posts: 37 Location: Tacoma, WA

right now I'm recompiling some KDE Arts library stuff (kde startup segfault, various apps not running because of missing libs, etc), and its taking *much* longer than it did originally. Is -march=xxxx known to take a lot longer than -mcpu=xxxx to operate?
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bushwacker · n00b Joined: 22 Feb 2004 Posts: 37 Location: Tacoma, WA

applying KERNEL_CFLAGS could be done by putting the following in /etc/make.conf, right?

KERNEL_CFLAGS="MY_COMPILE_PARAMETERS_HERE"

It seems to be going ok, but the new kernel 2.6.x build script doesn't show you the compilation data like the older kernels always did. Is there a way to show this info, too, BTW?

Thanks.
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Master_Of_Disaster · Posted: Sat May 15, 2004 12:18 am Post subject:

phreaqy · n00b Joined: 03 May 2004 Posts: 16 Location: Canada

Nobody seem to post if the kernel CFLAGS actually have a speed increase, so has anybody that tried this noticed a speed increase?
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bushwacker · n00b Joined: 22 Feb 2004 Posts: 37 Location: Tacoma, WA

The kernel seems to perform a bit better in terms of task switching/vm/cpu time/etc. I'm not sure if this is just because it's the 2.6x kernel as opposed to the 2.4 though. There seems to be a larger performance boost between stock 2.4 and stock 2.6 than between stock 2.6 and optimized 2.6 kernels.
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phreaqy · n00b Joined: 03 May 2004 Posts: 16 Location: Canada

Thanks Man

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bushwacker · n00b Joined: 22 Feb 2004 Posts: 37 Location: Tacoma, WA

NP. Let me know if you notice any positive results w/ this.
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dj_farid · l33t Joined: 14 Jun 2004 Posts: 613

Any results?

DRZIN · Tux's lil' helper Joined: 15 Feb 2006 Posts: 114

i belive all my issue that have been having is becouse my make.conf is not set up right please help!
what is differentes between menuconfig and make.conf
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