Re: [RFC PATCH] Flipped jump labels

From: Borislav Petkov
Date: Sun Aug 10 2014 - 12:08:12 EST

On Sun, Aug 10, 2014 at 05:45:15PM +0200, Ingo Molnar wrote:
> Indeed - but could we use that interface to cleanly expose the
> arch_static_branch_active() code you've written, or do we need new
> variants?

We could probably.

The thing is, if we want to do the _active thing, the whole jump labels
infrastructure would need to know about those, let's call them "new
types" because they'd need different handling when enabling - see
__jump_label_transform() in arch/x86/kernel/jump_label.c with all the
NOP checks.

Which begs the more important question: is adding those just to save us
a JMP penalty justify the additional code complexity. Frankly, I'm still
on the fence here and I'd rather do some perf measurements of a kernel
build with and without the JMP in native_sched_clock() to see whether it
is even noticeable or it disappears in the noise.

Because if it does disappear, the whole trouble is just for nothing.



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