[PATCH] Improve performance of libc locks

[Carlos O'Donell]

On 8/11/22 12:22, Wilco Dijkstra via Libc-alpha wrote:
> Improve performance of libc locks by adding a fast path for the
> single-threaded case.
> 
> On Neoverse V1, a loop using rand() improved 3.6 times. Multithreaded
> performance is unchanged.
> 
> Passes regress on AArch64, OK for commit?

This impacts all architectures.

Are you able to run the microbenchmarks to show that this improves one of them?

If you can, then we can at least ask the other machine maintainers to test the
patch shows gains there too. Conceptually I don't see why it wouldn't improve
the performance of all architectures, but having a baseline at the point of the
patch is good for recording the performance for future discussions.

If we don't have a benchmark that shows this specific base of ST vs MT and
internal __libc_lock_lock-locks then we should add one. Improving the internal
locking for our algorithms is always going to be a point of interest for IHVs.

Thanks.
 
> ---
> 
> diff --git a/sysdeps/nptl/libc-lockP.h b/sysdeps/nptl/libc-lockP.h
> index d3a6837fd212f3f5dfd80f46d0e9ce365042ae0c..ccdb11fee6f14069d0b936be93d0f0fa6d8bc30b 100644
> --- a/sysdeps/nptl/libc-lockP.h
> +++ b/sysdeps/nptl/libc-lockP.h
> @@ -108,7 +108,14 @@ _Static_assert (LLL_LOCK_INITIALIZER == 0, "LLL_LOCK_INITIALIZER != 0");
>  #define __libc_rwlock_fini(NAME) ((void) 0)
>  
>  /* Lock the named lock variable.  */
> -#define __libc_lock_lock(NAME) ({ lll_lock (NAME, LLL_PRIVATE); 0; })
> +#define __libc_lock_lock(NAME)						\
> + ({									\
> +    if (SINGLE_THREAD_P)						\
> +      (NAME) = LLL_LOCK_INITIALIZER_LOCKED;				\
> +    else								\
> +      lll_lock (NAME, LLL_PRIVATE);					\
> +    0;									\
> +  })
>  #define __libc_rwlock_rdlock(NAME) __pthread_rwlock_rdlock (&(NAME))
>  #define __libc_rwlock_wrlock(NAME) __pthread_rwlock_wrlock (&(NAME))
>  
> @@ -116,7 +123,14 @@ _Static_assert (LLL_LOCK_INITIALIZER == 0, "LLL_LOCK_INITIALIZER != 0");
>  #define __libc_lock_trylock(NAME) lll_trylock (NAME)
>  
>  /* Unlock the named lock variable.  */
> -#define __libc_lock_unlock(NAME) lll_unlock (NAME, LLL_PRIVATE)
> +#define __libc_lock_unlock(NAME)					\
> + ({									\
> +    if (SINGLE_THREAD_P)						\
> +      (NAME) = LLL_LOCK_INITIALIZER;					\
> +    else								\
> +      lll_unlock (NAME, LLL_PRIVATE);					\
> +    0;									\
> + })
>  #define __libc_rwlock_unlock(NAME) __pthread_rwlock_unlock (&(NAME))
>  
>  #if IS_IN (rtld)
> 
> 
> 


-- 
Cheers,
Carlos.