Re: [PATCH 00/33] Compile-time stack metadata validation

On Mon, Feb 15, 2016 at 10:31:34AM -0600, Josh Poimboeuf wrote:

On Fri, Feb 12, 2016 at 09:10:11PM +0100, Peter Zijlstra wrote:

quoted

On Fri, Feb 12, 2016 at 12:32:06PM -0600, Josh Poimboeuf wrote:

quoted

What I actually see in the listing is:

 	decl	__percpu_prefix:__preempt_count
 	je	1f:
	....
 1:
 	call	___preempt_schedule

So it puts the "call ___preempt_schedule" in the slow path.

Ah yes indeed. Same difference though.

quoted

I also don't see how that would be related to the use of the asm
statement in the __preempt_schedule() macro.  Doesn't the use of
unlikely() in preempt_enable() put the call in the slow path?

Sadly no, unlikely() and asm_goto don't work well together. But the slow
path or not isn't the reason we do the asm call thing.

quoted

  #define preempt_enable() \
  do { \
	  barrier(); \
	  if (unlikely(preempt_count_dec_and_test())) \
		  preempt_schedule(); \
  } while (0)

Also, why is the thunk needed?  Any reason why preempt_enable() can't be
called directly from C?

That would make the call-site save registers and increase the size of
every preempt_enable(). By using the thunk we can do callee saved
registers and avoid blowing up the call site.

So is the goal to optimize for size?  

General performance impact of preempt_enable().

If I replace the calls to
__preempt_schedule[_notrace]() with real C calls and remove the thunks,
it only adds about 2k to vmlinux.

That's less than I had expected, but probably still worth it.

And is that added text purely in the slow path? We really want to avoid
putting any more register pressure on the preempt_enable() call sites.
The single memop and Jcc is about as fast we can get and we spend quite
a bit of effort getting there.

There are two ways to fix the warnings:

1. get rid of the thunks and call the C functions directly; or

2. add the stack pointer to the asm() statement output operand list to
ensure a stack frame gets created in the caller function before the
call.  (Note this still allows the thunks to do callee saved registers.)

I like #1 better, but maybe I'm still missing the point of the thunks.

Ingo, Linus?
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