Re: [PATCH] Disable non-ABI-compliant optimisations for live patching

From: Jiri Kosina
Date: Thu Jun 23 2016 - 08:47:11 EST


On Thu, 23 Jun 2016, Jiri Kosina wrote:

> > I haven't looked at the fentry solution, but the code I'm involved in saves
> > the registers so that ftrace, live patch and friends can work freely. But
> > then it restores all regs and _then_ calls the replacement, so ftrace
> > saving all regs is no gain at all.
>
> You're right, thanks for bringing this up.
>
> In principle we should be able to modify the trampoline so that it
> performs its own register saving (in ftrace_regs_caller) and restoring
> (*), completely shielding the new function from any optimization gcc might
> have done on registers, shouldn't we?
>
> (*) we'll have to piggy-back on ftrace_epilogue on that, i.e. making the
> return to the original code go through trampoline as well (the same
> way graph tracer works)

Okay, after looking more about how ftrace implements the return
trampolines for graph caller, it'd be rather difficult to implement in a
way that we neither interfere with ftrace graph tracer (the
ftrace_ret_stack in task_struct) nor introduce a serious performance
overhead or stack usage pressure.

I am pretty sure the overhead we'd be adding would be much worse than just
really simply turning the IPA-RA off in CONFIG_LIVEPATCH-enabled kernels
is the easiest way to go.

After talking to Jan Hubicka, I'd actually suggest turning off most/all
the IPA optimizations; they are supposed to be of questionable benefit for
kernel anyway, and they might be causing serious issues for us.

I am planning to ask our performance team to measure the impact this'd
have.

Thanks,

--
Jiri Kosina
SUSE Labs