Use rpmbuild to compile kernel without re-compiling unchanged module

[paulhybryant]

Using the rpmbuild to compile the kernel, every time the patches will be applied to the source code. So even if no changes are made to the source code and no patches are added to kernel.spec, the whole kernel will still be recompiled, wasting a lot of time.

How can I avoid that?

Thank you very much!

[flyingfsck]

Noooo... Make will not recompile stuff that didn't change. It will check to see whether a file is the same and then move on to the next one. Maybe you are mistaking all these checks for an actual compile, or you did a 'make clean', or a 'configure', which will force a recompile of everything?

[vallimar]

Originally Posted by flyingfsck

Noooo... Make will not recompile stuff that didn't change. It will check to see whether a file is the same and then move on to the next one. Maybe you are mistaking all these checks for an actual compile, or you did a 'make clean', or a 'configure', which will force a recompile of everything?

They clearly stated their using rpmbuild, which doesn't work as you seem to think it does.

Originally Posted by paulhybryant

How can I avoid that?

Install the "ccache" package.

[paulhybryant]

hi, I make will not recompile unchanged module.

However, because the rpmbuild command reapply all the patches, though the file content is not changed, the modification date of the file is changed. Therefore make will think that all of them need to be recompilred.

I have tried ccache before, but it doesn't seem to work.

[vallimar]

It works well for me and I do kernel builds quite often.
So.. if you aren't making any changes, then why are you rebuilding the kernel with rpmbuild again?
Would be more efficient to just hang onto your previously generated rpms.

[paulhybryant]

I actually changed one file in the kernel.

But when I rebuild the kernel, it still takes the same amount of time as I built the kernel for the first time.

[leigh123linux]

Originally Posted by paulhybryant

I actually changed one file in the kernel.

But when I rebuild the kernel, it still takes the same amount of time as I built the kernel for the first time.

Try excluding the debugging

rpmbuild --rebuild --with baseonly  --without debuginfo --target=`uname -m` kernel-*.src.rpm

[paulhybryant]

yes, I excluded the debugging.

I think the problem is that every time rpmbuild will re-apply the patches, that is already applied in previous build.

Therefore even though the file content is the same, the modification time is changed. Thus the whole kernel is rebuilt.

[dd_wizard]

How about unpacking the src rpm, then building from the spec file after you make the changes. That will leave the previously compiled files in BUILD*.

dd_wizard

[paulhybryant]

Thanks dd_wizard.

However, then all the files are still compiled. It still takes a long time to build the kernel.
Is there anyway that I can save time by only compiling the files that is changed with rpmbuild?

[brunson]

With the src rpm installed use the --short-circuit option to rpmbuild to skip direction to the compile phase.

[paulhybryant]

Thanks brunson, that works!

Though the build time is still longer than I expected.

A fresh build of the kernel take me about 2 hours. And after changing two lines in mm/page_alloc.c,
it takes about 30 minutes to finish the build. Is this normal?

[brunson]

It depends on how the dependencies are set up in the make file. It still seems like a long time, but I haven't built a kernel in years.

[sudhanAnand]

Originally Posted by dd_wizard

How about unpacking the src rpm, then building from the spec file after you make the changes. That will leave the previously compiled files in BUILD*.
dd_wizard

I am trying to change few files in the kernel and build it again,
all the changes are getting rewritten by update patches, is there anyway i can
specify while i use rpmbuild to skip all the update patches?

thnx!