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    Subject[PATCH] x86: Use asm-goto to implement mutex fast path on x86-64
    Date
    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

    Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
    ---
    arch/x86/include/asm/mutex_64.h | 30 ++++++++++++++++++++++++++++++
    1 file changed, 30 insertions(+)

    diff --git a/arch/x86/include/asm/mutex_64.h b/arch/x86/include/asm/mutex_64.h
    index 68a87b0..ac3517b 100644
    --- a/arch/x86/include/asm/mutex_64.h
    +++ b/arch/x86/include/asm/mutex_64.h
    @@ -16,6 +16,20 @@
    *
    * Atomically decrements @v and calls <fail_fn> if the result is negative.
    */
    +#if defined(CC_HAVE_ASM_GOTO)
    +static inline void __mutex_fastpath_lock(atomic_t *v,
    + void (*fail_fn)(atomic_t *))
    +{
    + asm volatile goto(LOCK_PREFIX " decl %0\n"
    + " jns %l[exit]\n"
    + : : "m" (v->counter)
    + : "memory", "cc"
    + : exit);
    + fail_fn(v);
    +exit:
    + return;
    +}
    +#else
    #define __mutex_fastpath_lock(v, fail_fn) \
    do { \
    unsigned long dummy; \
    @@ -32,6 +46,7 @@ do { \
    : "rax", "rsi", "rdx", "rcx", \
    "r8", "r9", "r10", "r11", "memory"); \
    } while (0)
    +#endif

    /**
    * __mutex_fastpath_lock_retval - try to take the lock by moving the count
    @@ -59,6 +74,20 @@ static inline int __mutex_fastpath_lock_retval(atomic_t *count,
    *
    * Atomically increments @v and calls <fail_fn> if the result is nonpositive.
    */
    +#if defined(CC_HAVE_ASM_GOTO)
    +static inline void __mutex_fastpath_unlock(atomic_t *v,
    + void (*fail_fn)(atomic_t *))
    +{
    + asm volatile goto(LOCK_PREFIX " incl %0\n"
    + " jg %l[exit]\n"
    + : : "m" (v->counter)
    + : "memory", "cc"
    + : exit);
    + fail_fn(v);
    +exit:
    + return;
    +}
    +#else
    #define __mutex_fastpath_unlock(v, fail_fn) \
    do { \
    unsigned long dummy; \
    @@ -75,6 +104,7 @@ do { \
    : "rax", "rsi", "rdx", "rcx", \
    "r8", "r9", "r10", "r11", "memory"); \
    } while (0)
    +#endif

    #define __mutex_slowpath_needs_to_unlock() 1

    --
    1.7.9.5


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