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

[Denys Vlasenko]

On 04/10/2015 02:08 PM, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@xxxxxxxxxx> 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.
> 
> 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.

Looks like nobody objected. I think it's ok to submit
this patch for real.

> +	# Align jump targets to 1 byte, not the default 16 bytes:
> +        KBUILD_CFLAGS += -falign-jumps=1

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