Recompile and install only unmodified parts of kernel

titaniumdecoy · 01-01-2009, 03:15 AM

I am using these instructions to make and install a vanilla kernel with make-kpkg and dpkg.

However, it takes a very long time to compile the kernel.

Since the purpose of compiling my own kernel is to modify it, how can I only make and install those parts of the kernel and/or modules that have changed? Is there an update option for dpkg? Also, do I need to recompile and reinstall kernel-headers after each modification or just kernel-image?

Thanks.

Quakeboy02 · 01-01-2009, 12:39 PM

Quote:

Originally Posted by titaniumdecoy

I am using these instructions to make and install a vanilla kernel with make-kpkg and dpkg.

However, it takes a very long time to compile the kernel.

No it doesn't. If you have a quad-core processor, that is.

Here is the script that I use to compile kernels. Note the CONCURRENCY_LEVEL line. Even if you only have a single core, you can probably get away with using 2.

Code:

#!/bin/sh
export CONCURRENCY_LEVEL=4
make-kpkg clean
fakeroot make-kpkg --append_to_version -k8 --revision=0.1 --initrd kernel_image

Code:

Since the purpose of compiling my own kernel is to modify it, how can I only make and install those parts of the kernel and/or modules that have changed?

It depends on what you're trying to do. If you are modifying the code in a loadable module, you can recompile just the directory holding that module, and thus that module, by using something like this. Say you're working on the sata_nv driver. Of course you wouldn't be able to do this if you were booted from sata_nv, but it shows the general idea.

Code:

make M=drivers/ata
rmmod sata_nv
insmod drivers/ata/sata_nv.ko

Added: However, it's not a good idea to leave it like this once you get it working properly. You should remake the kernel and reinstall it.

Quote:

Is there an update option for dpkg?

You can see all the options by typing "man dpkg", but I don't think there is an update. Make-kpkg does an awful lot to make the .deb file.

Quote:

Also, do I need to recompile and reinstall kernel-headers after each modification or just kernel-image?

You don't have to install any kernel-headers package. Your kernel source is, by definition, the kernel-headers.

To replace a kernel with a new one of the same version, we normally would change the revision in the make command, and move the /lib/modules for the old one. To do so, generally follow this:

Code:

<make the new kernel file>
mv /lib/modules/kernelnumber  /lib/modules/kernelnumber.bak  (this is the one already installed)
dpkg -i newkernel

titaniumdecoy · 01-02-2009, 04:45 PM

Thanks, Quakeboy02. I'm still a little confused, though. Let's say I wanted to leave all the modules and headers intact but only update the kernel itself. You wrote that I could do this:

Code:

<make the new kernel file>
mv /lib/modules/kernelnumber  /lib/modules/kernelnumber.bak  (this is the one already installed)
dpkg -i newkernel

What do I replace <make the new kernel file> with? Thanks.

Quakeboy02 · 01-02-2009, 05:08 PM

Quote:

Originally Posted by titaniumdecoy

Thanks, Quakeboy02. I'm still a little confused, though. Let's say I wanted to leave all the modules and headers intact but only update the kernel itself. You wrote that I could do this:

Code:

<make the new kernel file>
mv /lib/modules/kernelnumber  /lib/modules/kernelnumber.bak  (this is the one already installed)
dpkg -i newkernel

What do I replace <make the new kernel file> with? Thanks.

OK, let's say that you have compiled kernel version 2.6.28 with the following commands:

Code:

export CONCURRENCY_LEVEL=2 (or whatever is appropriate for your system)
make-kpkg clean
fakeroot make-kpkg --append_to_version -k8 --revision=0.1 --initrd kernel_image

This gives you the output file: linux-image-2.6.28-k8_0.1_i386.deb (assuming x86_32) which you have installed with the following command, and subsequently booted:

Code:

sudo dpkg -i linux-image-2.6.28-k8_0.1_i386.deb

Now, you make some changes to the source and recompile it with this command:

Code:

export CONCURRENCY_LEVEL=4
make-kpkg clean   (I prefer to start from a cleaned source tree each time)
fakeroot make-kpkg --append_to_version -k8 --revision=0.2 --initrd kernel_image

Notice that I have changed the revision to 0.2 so that you have a new .deb file named: linux-image-2.6.28-k8_0.2_i386.deb. If you simply try to install this one over the running 2.6.28 (regardless of the revision number), the installer will complain that you may mess up some loadable modules. Instead, what you need to do is the following in this order:

Code:

sudo mv /lib/modules/2.6.28-k8   /lib/modules/2.6.28-k8.bak
sudo dpkg -i linux-image-2.6.28-k8_0.2_i386.deb

Then reboot. In practice, for a running kernel, I substitute `uname -r` for "2.6.28-k8" or whatever the kernel version to help avoid typos; i.e.

Code:

sudo mv /lib/modules/`uname -r` /lib/modules/`uname -r`.bak

It would appear that you have pulled the rug out from under yourself with the "mv" command and that this will kill your running kernel. But what actually happens is that the kernel is linked to the modules that are running, so it doesn't really matter whether you rename or even remove the subdirectory. What's important is that you get it out of the way so that dpkg can install the new subdirectory without any conflicts with modules.

Hope this helps.

titaniumdecoy · 01-03-2009, 03:04 AM

Recompiling all the modules each time takes forever. Is there any way to only remake only the kernel itself and leave /lib/modules/`uname -r` intact?

Quakeboy02 · 01-03-2009, 12:41 PM

Quote:

Originally Posted by titaniumdecoy

Recompiling all the modules each time takes forever. Is there any way to only remake only the kernel itself and leave /lib/modules/`uname -r` intact?

See my second paragraph of post #2. And disable any modules you don't need.

Have you tried increasing your concurrency level? On my quad core machine, a level of 4 brought the total compile time down to under 12 minutes. You can often get away with not doing the "clean", as well.