Messages in this thread | | | Date | Wed, 12 Jul 2017 21:27:50 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) |
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* Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Wed, Jul 12, 2017 at 10:27:10AM +0200, Ingo Molnar wrote: > > > Create a new "ORC" unwinder, enabled by CONFIG_ORC_UNWINDER, and plug it > > > into the x86 unwinder framework. Objtool is used to generate the ORC > > > debuginfo. The ORC debuginfo format is basically a simplified version > > > of DWARF CFI. More details below. > > > > BTW., we should perhaps consolidate our unwinder related Kconfig space, > > hierarchically: > > > > CONFIG_UNWINDER > > CONFIG_UNWINDER_ORC > > CONFIG_UNWINDER_FRAME_POINTERS > > > > Note that as a side effect it would be a valid small systems build option to have > > no unwinder at all, if CONFIG_EXPERT=y is set and such: !CONFIG_UNWINDER=n would > > be a sibling to !CONFIG_BUG. > > So is the idea that CONFIG_UNWINDER=n means "use the 'guess' unwinder"? > Or should it mean that the unwind API isn't available? > > Without frame pointers and orc, it defaults to the 'guess' unwinder, for > which the only overhead is a tiny amount of code. It's still > technically considered an unwinder because it plugs into the unwind > interfaces (unwind_start(), unwind_next_frame(), etc) and is used for > things like /proc/<pid>/stack. > > So I'm not really sure CONFIG_UNWINDER=n would make sense. Maybe there > should just be a multiple-choice where you have to choose one of > CONFIG_UNWINDER_{ORC,FRAME_POINTER,GUESS}.
Ok, you are right.
Maybe we could offer a menu of unwinders - i.e. make the whole Kconfig interface a bit nicer:
CONFIG_UNWINDER_FRAME_POINTER CONFIG_UNWINDER_ORC CONFIG_UNWINDER_GUESS
... or so?
Default would be the historic FRAME_POINTER, at least initially, I think.
I wouldn't mind making CONFIG_UNWINDER_ORC the new default either, due to the non-trivial speedup it offers - but maybe folks would object?
> > CONFIG_FRAME_POINTERS et al would be left for architectures where it has a meaning > > beyond backtrace generation. (Not sure whether there's any such architectures.) > > Well, on x86, hardened usercopy relies on frame pointers, but not the > unwinder. It does the frame pointer walk manually to avoid the full > unwinder overhead. See arch_within_stack_frames().
Oh well...
> Ok, how about: > > "Orc unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig > kernel) than DWARF eh_frame tables." > > (My previous 1MB number was from my distro-based config, and it also > forgot to take into account the fast lookup table (".orc_lookup")).
Sounds good to me!
Thanks,
Ingo
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