* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Thu, Jun 01, 2017 at 08:08:24AM +0200, Ingo Molnar wrote: > > > > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > > > Here's the contents of the undwarf.txt file which explains the 'why' in > > > more detail: > > > > Ok, so the code quality looks pretty convincing to me - the new core 'undwarf' > > unwinder code is a _lot_ more readable than any of the Dwarf based attempts > > before. > > > > That we control the debug info generation at build time is icing on the cake to > > me. > > > > One thing I'd like to see on the list of benefits side of the equation is a size > > comparison of kernel .text, with frame pointers vs. undwarf, on 64-bit kernels. > > Ok, will do a text size comparison. The only difficulty I encountered > there is that the 'size' tool considers the .undwarf section to be text > for some reason. So the "text" size grew considerably :-) One trick I sometimes use is to only size some of the key builtin.o files. > > Being able to generate more optimal code in the hottest code paths of the kernel > > is the _real_, primary upstream kernel benefit of a different debuginfo method - > > which has to be weighed against the pain of introducing a new unwinder. But this > > submission does not talk about that aspect at all, which should be fixed I think. > > Actually I devoted an entire one-sentence paragraph to performance in > the documentation: > > The simpler debuginfo format also enables the unwinder to be relatively > fast, which is important for perf and lockdep. > > But I'll try to highlight that a little more. That's not what I meant! The speedup comes from (hopefully) being able to disable CONFIG_FRAME_POINTER, which: - creates simpler/faster function prologues and epilogues - no managing of RBP needed - gives one more generic purpose register to work from. This matters less on 64-bit kernels but it's a small effect. I've seen numbers of 1-2% of instruction count reduction in common kernel workloads, which would be pretty significant on well cached workloads. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
