Re: [RFC PATCH] Flipped jump labels

[Jason Baron <jbaron@akamai.com>]

On 08/10/2014 12:07 PM, Borislav Petkov wrote:
> 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.
>
> Thanks.
>

That is correct. We don't currently support having the default
branch direction or the fast path be different from how the
'static_key' is initialized.

If I understand your use-case correctly,  you can't have the tsc
path be the default *before* tsc_init() is called?

If not, another thought on the implementation here might be
to re-visit the work the Steven Rostedt  proposed a while back,
to use jump instead of no-ops by default, and then convert
the jumps to no-ops in a post-processing phase in order to
reduce code size (by having 2-byte jumps for example on
x86). Potentially, we could then avoid converting some of the
jumps, if they didn't match the default branch direction.
See: http://www.serverphorums.com/read.php?12,759207

In that way there is no API change, we are just relaxing the
restriction that the default branch direction must match the
way that the keys are initialized.

Thanks,

-Jason