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

Paul E. McKenney
2015-04-10T14:10:20Z

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)
>