Re: [PATCH] x86: Align jump targets to 1 byte boundaries

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Apr 10, 2015 at 02:08:46PM +0200, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@kernel.org> wrote:
> 
> > So restructure the loop a bit, to get much tighter code:
> > 
> > 0000000000000030 <mutex_spin_on_owner.isra.5>:
> >   30:	55                   	push   %rbp
> >   31:	65 48 8b 14 25 00 00 	mov    %gs:0x0,%rdx
> >   38:	00 00
> >   3a:	48 89 e5             	mov    %rsp,%rbp
> >   3d:	48 39 37             	cmp    %rsi,(%rdi)
> >   40:	75 1e                	jne    60 <mutex_spin_on_owner.isra.5+0x30>
> >   42:	8b 46 28             	mov    0x28(%rsi),%eax
> >   45:	85 c0                	test   %eax,%eax
> >   47:	74 0d                	je     56 <mutex_spin_on_owner.isra.5+0x26>
> >   49:	f3 90                	pause
> >   4b:	48 8b 82 10 c0 ff ff 	mov    -0x3ff0(%rdx),%rax
> >   52:	a8 08                	test   $0x8,%al
> >   54:	74 e7                	je     3d <mutex_spin_on_owner.isra.5+0xd>
> >   56:	31 c0                	xor    %eax,%eax
> >   58:	5d                   	pop    %rbp
> >   59:	c3                   	retq
> >   5a:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)
> >   60:	b8 01 00 00 00       	mov    $0x1,%eax
> >   65:	5d                   	pop    %rbp
> >   66:	c3                   	retq
> 
> Btw., totally off topic, the following NOP caught my attention:
> 
> >   5a:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)
> 
> That's a dead NOP that boats the function a bit, added for the 16 byte 
> alignment of one of the jump targets.
> 
> I realize that x86 CPU manufacturers recommend 16-byte jump target 
> alignments (it's in the Intel optimization manual), but the cost of 
> that is very significant:
> 
>         text           data       bss         dec      filename
>     12566391        1617840   1089536    15273767      vmlinux.align.16-byte
>     12224951        1617840   1089536    14932327      vmlinux.align.1-byte
> 
> By using 1 byte jump target alignment (i.e. no alignment at all) we 
> get an almost 3% reduction in kernel size (!) - and a probably similar 
> reduction in I$ footprint.
> 
> So I'm wondering, is the 16 byte jump target optimization suggestion 
> really worth this price? The patch below boots fine and I've not 
> measured any noticeable slowdown, but I've not tried hard.

Good point, adding Josh Triplett on CC.  I suspect that he might be
interested.  ;-)

							Thanx, Paul

> Now, the usual justification for jump target alignment is the 
> following: with 16 byte instruction-cache cacheline sizes, if a 
> forward jump is aligned to cacheline boundary then prefetches will 
> start from a new cacheline.
> 
> But I think that argument is flawed for typical optimized kernel code 
> flows: forward jumps often go to 'cold' (uncommon) pieces of code, and 
> aligning cold code to cache lines does not bring a lot of advantages 
> (they are uncommon), while it causes collateral damage:
> 
>  - their alignment 'spreads out' the cache footprint, it shifts 
>    followup hot code further out
> 
>  - plus it slows down even 'cold' code that immediately follows 'hot' 
>    code (like in the above case), which could have benefited from the 
>    partial cacheline that comes off the end of hot code.
> 
> What do you guys think about this? I think we should seriously 
> consider relaxing our alignment defaults.
> 
> Thanks,
> 
> 	Ingo
> 
> ==================================>
> >From 5b83a095e1abdfee5c710c34a5785232ce74f939 Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@kernel.org>
> Date: Fri, 10 Apr 2015 13:50:05 +0200
> Subject: [PATCH] x86: Align jumps targets to 1 byte boundaries
> 
> Not-Yet-Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  arch/x86/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 5ba2d9ce82dc..0366d6b44a14 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -77,6 +77,9 @@ else
>          KBUILD_AFLAGS += -m64
>          KBUILD_CFLAGS += -m64
> 
> +	# Align jump targets to 1 byte, not the default 16 bytes:
> +        KBUILD_CFLAGS += -falign-jumps=1
> +
>          # Don't autogenerate traditional x87 instructions
>          KBUILD_CFLAGS += $(call cc-option,-mno-80387)
>          KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)
>