Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

From: Josh Poimboeuf
Date: Fri Jul 14 2017 - 13:18:24 EST

On Wed, Jul 12, 2017 at 09:27:50PM +0200, Ingo Molnar wrote:
> Maybe we could offer a menu of unwinders - i.e. make the whole Kconfig interface a
> bit nicer:
> ... or so?

So far I haven't been able to figure out how to make the above three
options into a multiple choice selection, such that allnoconfig selects
CONFIG_UNWINDER_GUESS and alldefconfig selects

I *think* I should be able to do it by setting the choice default to
FRAME_POINTER, and setting the 'allnoconfig_y' option for
UNWINDER_GUESS. But kconfig apparently doesn't support 'allnoconfig_y'
for choice selections yet. I may need to modify kconfig for that.

But IMO, this change can come later, and the current patches should be
fine to merge as-is. And it might make sense to delay such a patch
anyway, see below.

> 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?

Personally I wouldn't have an objection to making ORC the default,
though we should probably wait to give it some burn-in time first.

If we *do* decide to eventually make it the default, we could flip the
switch at the same time we introduced the multiple-choice config and
rename above. That way, users of "make oldconfig" would see the change
and would be encouraged to switch ORC.

> > > 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!

Ok, I'll post a v3.1 of patch 9 with the changed wording.