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:
>
> CONFIG_UNWINDER_FRAME_POINTER
> CONFIG_UNWINDER_ORC
> CONFIG_UNWINDER_GUESS
>
> ... 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
CONFIG_UNWINDER_FRAME_POINTER.
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.
--
Josh
