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I'm wanting to read and understand the Linux kernel's Memory Management (in particular defrag\compaction\migration).

So, I turn off optimization for size in .config (using make menuconfig of course) and compile...This leaves me with still an optimized kernel.

NOTE: When I say optimized kernel, I mean that when I use gdb and tell it next that it'll jump around. I don't want that, I want to be able to follow the code line by line just as I would with a simple hello world.

Next, I edit the Makefile and swap -O2 with -O0 and that causes things to break.

I found this, but I don't know what files I'll want because I don't know how far down the rabbit hole goes.

Is there a more generic option that I can use? I understand the concepts of memory compaction; however, I want to see where everything happens and how it exactly happens.

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  • How are you gdbing the kernel? Do you have kgdb support turned on? Commented Mar 19, 2015 at 17:46
  • @EugeneSh. I'm going through qemu -s in DEBUG_KERNEL is enabled. I'll enable kgdb and report the results. Commented Mar 19, 2015 at 17:51
  • @EugeneSh. Although I couldn't get a pipe working with qemu (gdb threw fits), there doesn't seem to be a difference. Commented Mar 19, 2015 at 20:36
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    While going through the question above, I'm suddenly curious, why Linux kernel has to be optimized for working properly? Commented Mar 20, 2015 at 4:09
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    Greg says it can't be done, thus you can't be done (in practice): lists.kernelnewbies.org/pipermail/kernelnewbies/2016-August/… :-) Related: unix.stackexchange.com/questions/153788/… Commented Feb 18, 2017 at 19:28

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You can't de-optimize the kernel. You might be able to de-optimize certain functions, like this:

void __attribute__((optimize("O0"))) foo(unsigned char data) { // unmodifiable compiler code }

But the kernel as a whole requires -O2 because the code itself is written with assumptions that certain functions will be optimized in a certain way.

Sorry, but you really will need to know the size of the rabbit hole you want to go down.

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Can we set optimization as -O1 instead -O0?
„But the kernel as a whole requires -O2 because the code itself is written with assumptions that certain functions will be optimized in a certain way.“ – That sounds ugly.
As of gcc v14.2, the documentation claims that -Og is better than -O0 for debugging. Even better: the (semi-random) function I tried to add this __attribute__ to does not compile with O0 but does with Og.

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