FW: ompd_get_thread_id in OMPD implementation

FW: ompd_get_thread_id in OMPD implementation

[Ahmed Sayed Mousse]

Sorry for the late reply.
I did check gomp_thread_self but I'm still not sure about what I should do,
maybe because I lack experience/knowledge.
Here is where my thinking is going right now and I hope you tell me if I'm
wrong.

in gomp_thread_to_pthread_t there are 4 possible outputs
1 - if LIBGOMP_USE_PTHREADS is enabled
 {
    first  pthread_self() if the thread calling is the same thread as the
function input.
    or  gomp_thread->handle in case GOMP_NEEDS_THREAD_HANDLE is enabled.
    or  pthread_self () + ((uintptr_t) input_thread - (uintptr_t)
calling_thread)
}
2 -if LIBGOMP_USE_PTHREADS not enabled
        - empty struct casted to a pthread_t
currently think i should check for the GOMP_NEED_THREAD_HANDLE, if it's
enabled i extract the pthread_t  from the gomp_thread handle given in the
function and return that.
If it's not enabled then I return an empty struct or an rc_unspported
return code.
Note:
    The openmp specification doesn't really tell me how things should be
done so I get confused a lot and I think I have a misunderstanding of the
function.
     I would appreciate it a lot if I get any directions to where I can
increase my knowledge around this part.

*From: *Ahmed Sayed Mousse <ahmedsayedmousse@gmail.com>
*Sent: *Wednesday, March 23, 2022 7:23 PM
*To: *gcc-help@gcc.gnu.org
*Cc: *jakub@redhat.com
*Subject: *ompd_get_thread_id in OMPD implementation



Hi everyone,

I was doing a research to help me implement the function
“ompd_get_thread_id” from the OpenMP API specification under section
20.5.5.5.



In this function I need to return a native thread identifier and from
research I found that it’s a “pthread_t” handle which exists inside a
struct named “gomp_thread” of the “libgomp.h” file. The problem is that
this handle isn’t always defined and to show what I mean look at the code
below.



struct gomp_thread

{

…..



#if defined(LIBGOMP_USE_PTHREADS) \

    && (!defined(HAVE_TLS) \

                || !defined(__GLIBC__) \

                || !defined(USING_INITIAL_EXEC_TLS))

#define GOMP_NEEDS_THREAD_HANDLE 1

  pthread_t handle;

#endif

};



I use a macro to calculate the offset of this handle and use the this
offset to get it from memory and I thought I would just check for
GOMP_NEEDS_THREAD_HANDLE before trying to calculate this offset but It
still causes errors and also if that handle isn’t defined then what should
I return as a native identifier?



Thanks for help.

Re: FW: ompd_get_thread_id in OMPD implementation

[Jakub Jelinek]

On Sat, Apr 09, 2022 at 12:38:11AM +0200, Ahmed Sayed Mousse wrote:
> Sorry for the late reply.
> I did check gomp_thread_self but I'm still not sure about what I should do,
> maybe because I lack experience/knowledge.
> Here is where my thinking is going right now and I hope you tell me if I'm
> wrong.

Sorry for the delay, I've been busy with GCC 12.

> in gomp_thread_to_pthread_t there are 4 possible outputs
> 1 - if LIBGOMP_USE_PTHREADS is enabled
>  {
>     first  pthread_self() if the thread calling is the same thread as the
> function input.
>     or  gomp_thread->handle in case GOMP_NEEDS_THREAD_HANDLE is enabled.
>     or  pthread_self () + ((uintptr_t) input_thread - (uintptr_t)
> calling_thread)
> }

ompd_get_thread_id is for mapping of the OMPD thread id to the native
thread id.  If LIBGOMP_USE_PTHREADS, we are using POSIX threads, so
OMPD_THREAD_ID_PTHREAD is what we want to provide.
If GOMP_NEEDS_THREAD_HANDLE, then we want to read the handle member for
it and return it.  Otherwise as the comment says, we optimize and
because we know that in the initial-exec TLS model &gomp_tls_data - pthread_self ()
is the same for each thread, we don't store the handle at all, so
ompd_get_thread_id instead needs to compute the bias.
If it is too hard to do it in libgompd.so alone, perhaps during
gompd_load libgomp.so.1 could compute it and store in some variable
that libgompd.so can then read.

> 2 -if LIBGOMP_USE_PTHREADS not enabled
>         - empty struct casted to a pthread_t
> currently think i should check for the GOMP_NEED_THREAD_HANDLE, if it's
> enabled i extract the pthread_t  from the gomp_thread handle given in the
> function and return that.
> If it's not enabled then I return an empty struct or an rc_unspported
> return code.
> Note:
>     The openmp specification doesn't really tell me how things should be
> done so I get confused a lot and I think I have a misunderstanding of the
> function.
>      I would appreciate it a lot if I get any directions to where I can
> increase my knowledge around this part.

If LIBGOMP_USE_PTHREADS is not enabled, then it is libgomp.a built for one
of the offloading targets, either NVPTX or GCN.  We then can't return
OMPD_THREAD_ID_PTHREAD, threads are numbered differently there, they are
either the CUDA threads or GCN threads.  But I think at least initially
we only build libgompd.so for the host so how exactly we OMPD offloading
should be postponed until after the host handling works.

	Jakub