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

[Thomas Schwinge <thomas@codesourcery.com>]

Hi Tobias!

On 2023-04-28T10:28:22+0200, Tobias Burnus <tobias@codesourcery.com> wrote:
> maybe I misunderstood your suggestion, but

Forst, note that those CUDA "Stream Memory Operations" are something that
I found by chance, and don't have any actual experience with.  I can't
seem to find a lot of documentation/usage of this API?

By the way, a similar thing also exists for AMD GPUs:
'hipStreamWaitValue32', etc.


> "Wait on a memory location"
> assumes that there will be a change – but if a target region happens to
> have no reverse offload, the memory location will never change, but
> still the target region should return to the host.

Oh indeed.  ;-) Details...

> What we would need: Wait on memory location – and return if either the
> kernel stopped *or* the memory location changed.

Or, have a way to "cancel", from the host, the 'cuStreamWaitValue32',
'cuStreamWaitValue64', after the actual 'target' kernel completed?

> My impression is that "return if the kernel stopped" is not really
> guaranteed. Of did I miss some fineprint?

No, you're right.  I suppose this is as designed: for example, generally,
there may be additional kernel launches, and the "wait" will then
eventually trigger.

Could we, after the actual 'target' kernel completed, issue a host-side
"write" ('cuStreamWriteValue32', 'cuStreamWriteValue64') to that memory
location, to signal end of processing for reverse offloads?

That is:

  - enqueue 'cuLaunchKernel'
  - enqueue 'cuStreamWriteValue' (to signal end of processing for reverse offloads)
  - loop on 'cuStreamWaitValue' (until end of processing for reverse offloads)


Grüße
 Thomas


> On 04.04.23 16:40, Thomas Schwinge wrote:
>> 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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> -----------------
> Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955
-----------------
Siemens Electronic Design Automation GmbH; Anschrift: Arnulfstraße 201, 80634 München; Gesellschaft mit beschränkter Haftung; Geschäftsführer: Thomas Heurung, Frank Thürauf; Sitz der Gesellschaft: München; Registergericht München, HRB 106955