Re: [Patch] libgomp/nvptx: Prepare for reverse-offload callback handling

[Thomas Schwinge <thomas@codesourcery.com>]

Hi!

During GCC/OpenMP/nvptx reverse offload investigations, about how to
replace the problematic global 'GOMP_REV_OFFLOAD_VAR', I may have found
something re:

On 2022-08-26T11:07:28+0200, Tobias Burnus <tobias@codesourcery.com> wrote:
> Better suggestions are welcome for the busy loop in
> libgomp/plugin/plugin-nvptx.c regarding the variable placement and checking
> its value.

> On the host side, the last address is checked - if fn_addr != NULL,
> it passes all arguments on to the generic (target.c) gomp_target_rev
> to do the actual offloading.
>
> CUDA does lockup when trying to copy data from the currently running
> stream; hence, a new stream is generated to do the memory copying.

> Future work for nvptx:
> * Adjust 'sleep', possibly [...]
>   to do shorter sleeps than usleep(1)?

... this busy loop.

Current 'libgomp/plugin/plugin-nvptx.c:GOMP_OFFLOAD_run':

    [...]
      if (reverse_offload)
        CUDA_CALL_ASSERT (cuStreamCreate, &copy_stream, CU_STREAM_NON_BLOCKING);
      r = CUDA_CALL_NOCHECK (cuLaunchKernel, function, teams, 1, 1,
                             32, threads, 1, 0, NULL, NULL, config);
      if (r != CUDA_SUCCESS)
        GOMP_PLUGIN_fatal ("cuLaunchKernel error: %s", cuda_error (r));
      if (reverse_offload)
        while (true)
          {
            r = CUDA_CALL_NOCHECK (cuStreamQuery, NULL);
            if (r == CUDA_SUCCESS)
              break;
            if (r == CUDA_ERROR_LAUNCH_FAILED)
              GOMP_PLUGIN_fatal ("cuStreamQuery error: %s %s\n", cuda_error (r),
                                 maybe_abort_msg);
            else if (r != CUDA_ERROR_NOT_READY)
              GOMP_PLUGIN_fatal ("cuStreamQuery error: %s", cuda_error (r));

            if (__atomic_load_n (&ptx_dev->rev_data->fn, __ATOMIC_ACQUIRE) != 0)
              {
                struct rev_offload *rev_data = ptx_dev->rev_data;
                GOMP_PLUGIN_target_rev (rev_data->fn, rev_data->mapnum,
                                        rev_data->addrs, rev_data->sizes,
                                        rev_data->kinds, rev_data->dev_num,
                                        rev_off_dev_to_host_cpy,
                                        rev_off_host_to_dev_cpy, copy_stream);
                CUDA_CALL_ASSERT (cuStreamSynchronize, copy_stream);
                __atomic_store_n (&rev_data->fn, 0, __ATOMIC_RELEASE);
              }
            usleep (1);
          }
      else
        r = CUDA_CALL_NOCHECK (cuCtxSynchronize, );
      if (reverse_offload)
        CUDA_CALL_ASSERT (cuStreamDestroy, copy_stream);
    [...]

Instead of this 'while (true)', 'usleep (1)' loop, shouldn't we be able
to use "Stream Memory Operations",
<https://docs.nvidia.com/cuda/cuda-driver-api/group__CUDA__MEMOP.html>,
that allow to "Wait on a memory location", "until the given condition on
the memory is satisfied"?

For reference, current 'libgomp/config/nvptx/target.c:GOMP_target_ext':

    [...]
      GOMP_REV_OFFLOAD_VAR->mapnum = mapnum;
      GOMP_REV_OFFLOAD_VAR->addrs = (uint64_t) hostaddrs;
      GOMP_REV_OFFLOAD_VAR->sizes = (uint64_t) sizes;
      GOMP_REV_OFFLOAD_VAR->kinds = (uint64_t) kinds;
      GOMP_REV_OFFLOAD_VAR->dev_num = GOMP_ADDITIONAL_ICVS.device_num;

      /* Set 'fn' to trigger processing on the host; wait for completion,
         which is flagged by setting 'fn' back to 0 on the host.  */
      uint64_t addr_struct_fn = (uint64_t) &GOMP_REV_OFFLOAD_VAR->fn;
    #if __PTX_SM__ >= 700
      asm volatile ("st.global.release.sys.u64 [%0], %1;"
                    : : "r"(addr_struct_fn), "r" (fn) : "memory");
    #else
      __sync_synchronize ();  /* membar.sys */
      asm volatile ("st.volatile.global.u64 [%0], %1;"
                    : : "r"(addr_struct_fn), "r" (fn) : "memory");
    #endif

    #if __PTX_SM__ >= 700
      uint64_t fn2;
      do
        {
          asm volatile ("ld.acquire.sys.global.u64 %0, [%1];"
                        : "=r" (fn2) : "r" (addr_struct_fn) : "memory");
        }
      while (fn2 != 0);
    #else
      /* ld.global.u64 %r64,[__gomp_rev_offload_var];
         ld.u64 %r36,[%r64];
         membar.sys;  */
      while (__atomic_load_n (&GOMP_REV_OFFLOAD_VAR->fn, __ATOMIC_ACQUIRE) != 0)
        ;  /* spin  */
    #endif
    [...]


Grüße
 Thomas
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