Re: [PATCH 2/2] PowerPC: libc single-thread lock optimization

[Torvald Riegel <triegel at redhat dot com>]

On Wed, 2014-04-30 at 10:57 -0300, Adhemerval Zanella wrote:
> This patch adds a single-thread optimization for locks used within
> libc.so.  For each lock operations it checks it the process has already
> spawned one thread and if not use non-atomic operations.  Other libraries
> (libpthread.so for instance) are unaffected by this change.
> 
> This is a respin on my first patch to add such optimization, but now the
> code is focused only on lowlevellock.h, the atomic.h is untouched.
> 
> Tested on powerpc32 and powerpc64.
> 
> --
> 
>         * nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
>         (__lll_is_single_thread): New macro to check if process has spawned
>         any threads.
>         (__lll_robust_trylock): Add optimization to avoid atomic operations in
>         single thread case.
>         (lll_lock): Likewise.
>         (lll_robust_lock): Likewise.
>         (lll_cond_lock): Likewise.
>         (lll_robust_cond_lock): Likewise.
>         (lll_timedlock): Likewise.
>         (lll_robust_timedlock): Likewise.
>         (lll_unlock): Likewise.
>         (lll_robust_unlock): Likewise.
> 
> ---
> 
> diff --git a/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h b/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
> index ab92c3f..38529a4 100644
> --- a/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
> +++ b/nptl/sysdeps/unix/sysv/linux/powerpc/lowlevellock.h
> @@ -76,6 +76,16 @@
>  # endif
>  #endif
>  
> +/* For internal libc.so lock calls in single-thread process we use normal
> +   load/stores.  */
> +#if !defined NOT_IN_libc || defined UP
> +# define __lll_is_single_thread                                                      \
> +  __glibc_likely (THREAD_GETMEM (THREAD_SELF, header.multiple_threads) == 0)

I disagree that single-threaded execution should be the likely case.
There is a large body of existing single-threaded code.  But what's the
microbenchmarks or data that we base this decision on?

> +#else
> +# define __lll_is_single_thread (0)
> +#endif
> +
> +
>  #define lll_futex_wait(futexp, val, private) \
>    lll_futex_timed_wait (futexp, val, NULL, private)
>  
> @@ -205,7 +215,9 @@
>  /* Set *futex to ID if it is 0, atomically.  Returns the old value */
>  #define __lll_robust_trylock(futex, id) \
>    ({ int __val;                                                                      \
> -     __asm __volatile ("1:     lwarx   %0,0,%2" MUTEX_HINT_ACQ "\n"          \
> +     if (!__lll_is_single_thread)                                            \
> +       __asm __volatile (                                                    \
> +                      "1:      lwarx   %0,0,%2" MUTEX_HINT_ACQ "\n"          \
>                        "        cmpwi   0,%0,0\n"                             \
>                        "        bne     2f\n"                                 \
>                        "        stwcx.  %3,0,%2\n"                            \
> @@ -214,6 +226,12 @@
>                        : "=&r" (__val), "=m" (*futex)                         \
>                        : "r" (futex), "r" (id), "m" (*futex)                  \
>                        : "cr0", "memory");                                    \
> +     else                                                                    \
> +       {                                                                     \
> +        __val = *futex;                                                      \
> +        if (__val == 0)                                                      \
> +          *futex = id;                                                       \
> +       }                                                                     \
>       __val;                                                                  \
>    })

Conceptually, you can safely use trylock in signal handlers, because
it's not a blocking operation.  And this is used for normal and robust
locks.  I haven't checked all the trylock code and all uses to see
whether it is indeed AS-Safe, but I'm pretty sure this change is wrong.
At the very least, it doesn't document that trylock is now AS-Unsafe.

Regarding the robust locks case: What is using it outside of nptl?  And
if so, what's the point of using an explicitly robust lock if there's no
concurrency?  Who's going to clean up afterwards?
 
> @@ -237,8 +255,16 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
>  #define lll_lock(lock, private) \
>    (void) ({                                                                  \
>      int *__futex = &(lock);                                                  \
> -    if (__builtin_expect (atomic_compare_and_exchange_val_acq (__futex, 1, 0),\
> -                         0) != 0)                                            \
> +    int __tmp;                                                               \
> +    if (!__lll_is_single_thread)                                             \
> +      __tmp = atomic_compare_and_exchange_val_acq (__futex, 1, 0);           \
> +    else                                                                     \
> +      {                                                                              \
> +       __tmp = *__futex;                                                     \
> +       if (__tmp == 0)                                                       \
> +         *__futex = 1;                                                       \
> +      }                                                                              \
> +    if (__builtin_expect (__tmp, 0) != 0)                                    \
>        {                                                                              \
>         if (__builtin_constant_p (private) && (private) == LLL_PRIVATE)       \
>           __lll_lock_wait_private (__futex);                                  \

This is obviously wrong for anything in nptl because you don't check
private.  You defined lll_is_single_thread so that it's always 0 in
nptl, so this works -- but it's not increasing the clarity of the code.

You also assume that nobody outside of nptl will call lll_lock with
private not set to LLL_PRIVATE.  This seems error-prone.  Or can you
show why that's fine?

Also, you add this code to all low-level locks independently of whether
they are actually used outside of nptl or not.  Have you looked at
libc_lock.h?  I'm not familiar with how non-nptl code uses the locks,
but it seems it's calling nptl code, so the change wouldn't have any
effect.

Finally, for a change as this whose effect on performance is
non-obvious, I think we really need performance measurements.  We need
to have microbenchmarks so that we can track the change of performance.