Re: [PATCH v7] libgfortran: Replace mutex with rwlock

[Lipeng Zhu <lipeng.zhu@intel.com>]

On 2023/12/14 23:50, Richard Earnshaw (lists) wrote:
> On 09/12/2023 15:39, Lipeng Zhu wrote:
>> This patch try to introduce the rwlock and split the read/write to
>> unit_root tree and unit_cache with rwlock instead of the mutex to
>> increase CPU efficiency. In the get_gfc_unit function, the percentage
>> to step into the insert_unit function is around 30%, in most instances,
>> we can get the unit in the phase of reading the unit_cache or unit_root
>> tree. So split the read/write phase by rwlock would be an approach to
>> make it more parallel.
>>
>> BTW, the IPC metrics can gain around 9x in our test
>> server with 220 cores. The benchmark we used is
>> https://github.com/rwesson/NEAT
>>
>> libgcc/ChangeLog:
>>
>> 	* gthr-posix.h (__GTHREAD_RWLOCK_INIT): New macro.
>> 	(__gthrw): New function.
>> 	(__gthread_rwlock_rdlock): New function.
>> 	(__gthread_rwlock_tryrdlock): New function.
>> 	(__gthread_rwlock_wrlock): New function.
>> 	(__gthread_rwlock_trywrlock): New function.
>> 	(__gthread_rwlock_unlock): New function.
>>
>> libgfortran/ChangeLog:
>>
>> 	* io/async.c (DEBUG_LINE): New macro.
>> 	* io/async.h (RWLOCK_DEBUG_ADD): New macro.
>> 	(CHECK_RDLOCK): New macro.
>> 	(CHECK_WRLOCK): New macro.
>> 	(TAIL_RWLOCK_DEBUG_QUEUE): New macro.
>> 	(IN_RWLOCK_DEBUG_QUEUE): New macro.
>> 	(RDLOCK): New macro.
>> 	(WRLOCK): New macro.
>> 	(RWUNLOCK): New macro.
>> 	(RD_TO_WRLOCK): New macro.
>> 	(INTERN_RDLOCK): New macro.
>> 	(INTERN_WRLOCK): New macro.
>> 	(INTERN_RWUNLOCK): New macro.
>> 	* io/io.h (struct gfc_unit): Change UNIT_LOCK to UNIT_RWLOCK in
>> 	a comment.
>> 	(unit_lock): Remove including associated internal_proto.
>> 	(unit_rwlock): New declarations including associated internal_proto.
>> 	(dec_waiting_unlocked): Use WRLOCK and RWUNLOCK on unit_rwlock
>> 	instead of __gthread_mutex_lock and __gthread_mutex_unlock on
>> 	unit_lock.
>> 	* io/transfer.c (st_read_done_worker): Use WRLOCK and RWUNLOCK on
>> 	unit_rwlock instead of LOCK and UNLOCK on unit_lock.
>> 	(st_write_done_worker): Likewise.
>> 	* io/unit.c: Change UNIT_LOCK to UNIT_RWLOCK in 'IO locking rules'
>> 	comment. Use unit_rwlock variable instead of unit_lock variable.
>> 	(get_gfc_unit_from_unit_root): New function.
>> 	(get_gfc_unit): Use RDLOCK, WRLOCK and RWUNLOCK on unit_rwlock
>> 	instead of LOCK and UNLOCK on unit_lock.
>> 	(close_unit_1): Use WRLOCK and RWUNLOCK on unit_rwlock instead of
>> 	LOCK and UNLOCK on unit_lock.
>> 	(close_units): Likewise.
>> 	(newunit_alloc): Use RWUNLOCK on unit_rwlock instead of UNLOCK on
>> 	unit_lock.
>> 	* io/unix.c (find_file): Use RDLOCK and RWUNLOCK on unit_rwlock
>> 	instead of LOCK and UNLOCK on unit_lock.
>> 	(flush_all_units): Use WRLOCK and RWUNLOCK on unit_rwlock instead
>> 	of LOCK and UNLOCK on unit_lock.
>>
> 
> It looks like this has broken builds on arm-none-eabi when using newlib:
> 
> In file included from /work/rearnsha/gnusrc/nightly/gcc-cross/master/libgfortran
> /runtime/error.c:27:
> /work/rearnsha/gnusrc/nightly/gcc-cross/master/libgfortran/io/io.h: In function
> ‘dec_waiting_unlocked’:
> /work/rearnsha/gnusrc/nightly/gcc-cross/master/libgfortran/io/io.h:1023:3: error
> : implicit declaration of function ‘WRLOCK’ [-Wimplicit-function-declaration]
>   1023 |   WRLOCK (&unit_rwlock);
>        |   ^~~~~~
> /work/rearnsha/gnusrc/nightly/gcc-cross/master/libgfortran/io/io.h:1025:3: error
> : implicit declaration of function ‘RWUNLOCK’ [-Wimplicit-function-declaration]
>   1025 |   RWUNLOCK (&unit_rwlock);
>        |   ^~~~~~~~
> 
> 
> R.

Hi Richard,

The root cause is that the macro WRLOCK and RWUNLOCK are not defined in 
io.h. The reason of x86 platform not failed is that 
HAVE_ATOMIC_FETCH_ADD is defined then caused above macros were never 
been used. Code logic show as below:
#ifdef HAVE_ATOMIC_FETCH_ADD
   (void) __atomic_fetch_add (&u->waiting, -1, __ATOMIC_RELAXED);
#else
   WRLOCK (&unit_rwlock);
   u->waiting--;
   RWUNLOCK (&unit_rwlock);
#endif

I just draft a patch try to fix this bug, because I didn't have arm 
platform, would you help to validate if it was fixed on arm platform?

diff --git a/libgfortran/io/io.h b/libgfortran/io/io.h
index 15daa0995b1..c7f0f7d7d9e 100644
--- a/libgfortran/io/io.h
+++ b/libgfortran/io/io.h
@@ -1020,9 +1020,15 @@ dec_waiting_unlocked (gfc_unit *u)
  #ifdef HAVE_ATOMIC_FETCH_ADD
    (void) __atomic_fetch_add (&u->waiting, -1, __ATOMIC_RELAXED);
  #else
-  WRLOCK (&unit_rwlock);
+#ifdef __GTHREAD_RWLOCK_INIT
+  __gthread_rwlock_wrlock (&unit_rwlock);
+  u->waiting--;
+  __gthread_rwlock_unlock (&unit_rwlock);
+#else
+  __gthread_mutex_lock (&unit_rwlock);
    u->waiting--;
-  RWUNLOCK (&unit_rwlock);
+  __gthread_mutex_unlock (&unit_rwlock);
+#endif
  #endif
  }


Lipeng Zhu