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Re: [PATCH 2/2] PowerPC: libc single-thread lock optimization
- From: Torvald Riegel <triegel at redhat dot com>
- To: Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>
- Cc: "GNU C. Library" <libc-alpha at sourceware dot org>
- Date: Fri, 02 May 2014 17:42:50 +0200
- Subject: Re: [PATCH 2/2] PowerPC: libc single-thread lock optimization
- Authentication-results: sourceware.org; auth=none
- References: <5361013D dot 9020005 at linux dot vnet dot ibm 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.