linux-kernel - Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64

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Date:	Fri, 28 Jun 2013 13:19:48 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Wedson Almeida Filho <wedsonaf@...il.com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Use asm-goto to implement mutex fast path on x86-64


* Wedson Almeida Filho <wedsonaf@...il.com> wrote:

> The new implementation allows the compiler to better optimize the code; the
> original implementation is still used when the kernel is compiled with older
> versions of gcc that don't support asm-goto.
> 
> Compiling with gcc 4.7.3, the original mutex_lock() is 60 bytes with the fast
> path taking 16 instructions; the new mutex_lock() is 42 bytes, with the fast
> path taking 12 instructions.
> 
> The original mutex_unlock() is 24 bytes with the fast path taking 7
> instructions; the new mutex_unlock() is 25 bytes (because the compiler used
> a 2-byte ret) with the fast path taking 4 instructions.
> 
> The two versions of the functions are included below for reference.
> 
> Old:
> ffffffff817742a0 <mutex_lock>:
> ffffffff817742a0:       55                      push   %rbp
> ffffffff817742a1:       48 89 e5                mov    %rsp,%rbp
> ffffffff817742a4:       48 83 ec 10             sub    $0x10,%rsp
> ffffffff817742a8:       48 89 5d f0             mov    %rbx,-0x10(%rbp)
> ffffffff817742ac:       48 89 fb                mov    %rdi,%rbx
> ffffffff817742af:       4c 89 65 f8             mov    %r12,-0x8(%rbp)
> ffffffff817742b3:       e8 28 15 00 00          callq  ffffffff817757e0 <_cond_resched>
> ffffffff817742b8:       48 89 df                mov    %rbx,%rdi
> ffffffff817742bb:       f0 ff 0f                lock decl (%rdi)
> ffffffff817742be:       79 05                   jns    ffffffff817742c5 <mutex_lock+0x25>
> ffffffff817742c0:       e8 cb 04 00 00          callq  ffffffff81774790 <__mutex_lock_slowpath>
> ffffffff817742c5:       65 48 8b 04 25 c0 b7    mov    %gs:0xb7c0,%rax
> ffffffff817742cc:       00 00
> ffffffff817742ce:       4c 8b 65 f8             mov    -0x8(%rbp),%r12
> ffffffff817742d2:       48 89 43 18             mov    %rax,0x18(%rbx)
> ffffffff817742d6:       48 8b 5d f0             mov    -0x10(%rbp),%rbx
> ffffffff817742da:       c9                      leaveq
> ffffffff817742db:       c3                      retq
> 
> ffffffff81774250 <mutex_unlock>:
> ffffffff81774250:       55                      push   %rbp
> ffffffff81774251:       48 c7 47 18 00 00 00    movq   $0x0,0x18(%rdi)
> ffffffff81774258:       00
> ffffffff81774259:       48 89 e5                mov    %rsp,%rbp
> ffffffff8177425c:       f0 ff 07                lock incl (%rdi)
> ffffffff8177425f:       7f 05                   jg     ffffffff81774266 <mutex_unlock+0x16>
> ffffffff81774261:       e8 ea 04 00 00          callq  ffffffff81774750 <__mutex_unlock_slowpath>
> ffffffff81774266:       5d                      pop    %rbp
> ffffffff81774267:       c3                      retq
> 
> New:
> ffffffff81774920 <mutex_lock>:
> ffffffff81774920:       55                      push   %rbp
> ffffffff81774921:       48 89 e5                mov    %rsp,%rbp
> ffffffff81774924:       53                      push   %rbx
> ffffffff81774925:       48 89 fb                mov    %rdi,%rbx
> ffffffff81774928:       e8 a3 0e 00 00          callq  ffffffff817757d0 <_cond_resched>
> ffffffff8177492d:       f0 ff 0b                lock decl (%rbx)
> ffffffff81774930:       79 08                   jns    ffffffff8177493a <mutex_lock+0x1a>
> ffffffff81774932:       48 89 df                mov    %rbx,%rdi
> ffffffff81774935:       e8 16 fe ff ff          callq  ffffffff81774750 <__mutex_lock_slowpath>
> ffffffff8177493a:       65 48 8b 04 25 c0 b7    mov    %gs:0xb7c0,%rax
> ffffffff81774941:       00 00
> ffffffff81774943:       48 89 43 18             mov    %rax,0x18(%rbx)
> ffffffff81774947:       5b                      pop    %rbx
> ffffffff81774948:       5d                      pop    %rbp
> ffffffff81774949:       c3                      retq
> 
> ffffffff81774730 <mutex_unlock>:
> ffffffff81774730:       48 c7 47 18 00 00 00    movq   $0x0,0x18(%rdi)
> ffffffff81774737:       00
> ffffffff81774738:       f0 ff 07                lock incl (%rdi)
> ffffffff8177473b:       7f 0a                   jg     ffffffff81774747 <mutex_unlock+0x17>
> ffffffff8177473d:       55                      push   %rbp
> ffffffff8177473e:       48 89 e5                mov    %rsp,%rbp
> ffffffff81774741:       e8 aa ff ff ff          callq  ffffffff817746f0 <__mutex_unlock_slowpath>
> ffffffff81774746:       5d                      pop    %rbp
> ffffffff81774747:       f3 c3                   repz retq

Very nice!

One detail:

> +#if defined(CC_HAVE_ASM_GOTO)
> +#if defined(CC_HAVE_ASM_GOTO)

Please change these to #ifdef.

Thanks,

	Ingo
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