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

[Vaseeharan Vinayagamoorthy <Vaseeharan.Vinayagamoorthy@arm.com>]

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Hi Lipeng,

It looks like your draft patch to fix the builds for arm-none-eabi target is not merged yet, because our arm-none-eabi builds are still broken. Are you waiting for additional information, or would you be able to fix this issue?

Kind regards,
Vasee
________________________________
From: Richard Earnshaw <Richard.Earnshaw@foss.arm.com>
Sent: 15 December 2023 19:23
To: Lipeng Zhu <lipeng.zhu@intel.com>; Richard Earnshaw <Richard.Earnshaw@arm.com>; jakub@redhat.com <jakub@redhat.com>
Cc: fortran@gcc.gnu.org <fortran@gcc.gnu.org>; gcc-patches@gcc.gnu.org <gcc-patches@gcc.gnu.org>; hongjiu.lu@intel.com <hongjiu.lu@intel.com>; pan.deng@intel.com <pan.deng@intel.com>; rep.dot.nop@gmail.com <rep.dot.nop@gmail.com>; tianyou.li@intel.com <tianyou.li@intel.com>; tkoenig@netcologne.de <tkoenig@netcologne.de>; wangyang.guo@intel.com <wangyang.guo@intel.com>
Subject: Re: [PATCH v7] libgfortran: Replace mutex with rwlock



On 15/12/2023 11:31, Lipeng Zhu wrote:
>
>
> 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

Hi Lipeng,

Thanks for the quick reply.  I can confirm that with the above change
the bootstrap failure is fixed.  However, this shouldn't be considered a
formal review; libgfortran is not really my area.

I'll be away now until January 2nd.

Richard.