On Mon 2016-12-19 11:25:49, Josh Poimboeuf wrote:

On Mon, Dec 19, 2016 at 05:25:19PM +0100, Miroslav Benes wrote:

quoted

On Thu, 8 Dec 2016, Josh Poimboeuf wrote:

quoted

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 215612c..b4a6663 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig

@@ -155,6 +155,7 @@ config X86
 	select HAVE_PERF_REGS
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_REGS_AND_STACK_ACCESS_API
+	select HAVE_RELIABLE_STACKTRACE		if X86_64 && FRAME_POINTER && STACK_VALIDATION

Tests to measure possible performance penalty of frame pointers were done 
by Mel Gorman. The outcome was quite clear. There IS a measurable 
impact. The percentage depends on the workflow but I think it is safe to 
say that FP usually takes 5-10 percents.

If my understanding is correct there is no single culprit. Register 
pressure is definitely not a problem. We ran simple benchmarks while 
taking a register away from GCC (RBP or a common one). The impact is a 
combination of more cacheline pressure, more memory accesses and the fact 
that the kernel contains a lot of small functions.

Thus, I think that DWARF should be the way to go here.

Other than that the patch looks good to me.

I agree that DWARF is generally a good idea, and I'm working toward it.
However there's still quite a bit of work to get there.

For this consistency model to work with DWARF on x86, we would need:

1) a reliable x86 DWARF unwinder with Linus's blessing
2) objtool DWARF support (I'm working on this at the moment)
3) probably some kind of runtime NMI stack checking feature to
   complement objtool, along with a lot of burn time to ensure there are
   no issues, particularly in entry code

Could you please provide more details about this NMI stack checking?
What is it supposed to protect that objtool could not?
Will it run regularly or will it be just a random check?

The other points are obvious. But I do not know what to
think about this NMI thing. And so I am curious :-)

4) port save_stack_trace_tsk_reliable() to work with DWARF

Best Regards,
Petr