[CUDA] floating-point exception in cuda_piextUSMSharedAlloc

[jinz2014]

https://github.com/jeffhammond/dpcpp-tutorial/blob/master/saxpy-usm.cc

Debugging the above program shows the following:

Thread 1 "a.out" received signal SIGFPE, Arithmetic exception.
0x00007ffff62eae0b in cuda_piextUSMSharedAlloc () from /home/cc/sycl_workspace/build/install/lib/libpi_cuda.so
(gdb) bt
#0 0x00007ffff62eae0b in cuda_piextUSMSharedAlloc () from /home/cc/sycl_workspace/build/install/lib/libpi_cuda.so
#1 0x00007ffff72002cb in cl::sycl::detail::usm::alignedAlloc(unsigned long, unsigned long, cl::sycl::context const&, cl::sycl::device const&, cl::sycl::usm::alloc) () from /home/cc/sycl_workspace/build/install/lib/libsycl.so
#2 0x00000000004027ee in main ()

[romanovvlad]

Looks like the crash happens because alignment is 0 on the following line
https://github.com/intel/llvm/blob/sycl/sycl/plugins/cuda/pi_cuda.cpp#L3430 :
assert(reinterpret_cast<std::uintptr_t>(*result_ptr) % alignment == 0);

The OpenCL extension for USM (https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/USM/cl_intel_unified_shared_memory.asciidoc) says:

alignment is the minimum alignment in bytes for the requested host allocation. It must be a power of two and must be equal to or smaller than the size of the largest data type supported by any OpenCL device in context. If alignment is 0, a default alignment will be used that is equal to the size of the largest data type supported by any OpenCL device in context.

I believe largest type is double16.

The SYCL extension for USM(https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/USM/USM.adoc) says:

size_t alignment - specifies the byte alignment. Must be a valid alignment supported by the implementation.

So, @fwyzard

1. Cannot find info about alignment, for example, for cuMemAllocHost, do you know what alignment this function guarantees?
2. Shouldn't this assert be a runtime check instead?
3. Is it OK to change alignment to 1 if it is 0?

@jbrodman What should an implementation do if an alignment passed by user is not a valid alignment supported by the implementation?

[fwyzard]

hi @romanovvlad

So, @fwyzard

1. Cannot find info about alignment, for example, for [cuMemAllocHost](http://developer.download.nvidia.com/compute/cuda/3_0/toolkit/docs/online/group__CUMEM_g707085f1c7b0235429766a0cbd5b9cec.html#g707085f1c7b0235429766a0cbd5b9cec), do you know what alignment this function guarantees?

Neither can I.
The latest documentation doesn't say anything about alignment of the memory returned by cuMemHostAlloc()/cuMemAllocHost().
While about cuMemAlloc()/cuMemAllocManaged() it explicitly says

The allocated memory is suitably aligned for any kind of variable.

Let me try some empirical tests and/or asking NVIDIA about it.

---

2. Shouldn't this assert be a runtime check instead?

Do you mean it should raise an exception rather than aborting ?

---

3. Is it OK to change alignment to 1 if it is 0?

I would take alignment == 0 as requiring no specific alignment, and just ignore it.
That is, the check could become

 assert((alignment == 0) or (reinterpret_cast<std::uintptr_t>(*result_ptr) % alignment == 0));

[fwyzard]

From a quick test, it looks like the memory returned by cuMemAllocHost() is aligned to 512 (0x200) bytes.

[fwyzard]

Can you check if #1577 fixes the exception ?

[jbrodman]

hi @romanovvlad

So, @fwyzard

1. Cannot find info about alignment, for example, for [cuMemAllocHost](http://developer.download.nvidia.com/compute/cuda/3_0/toolkit/docs/online/group__CUMEM_g707085f1c7b0235429766a0cbd5b9cec.html#g707085f1c7b0235429766a0cbd5b9cec), do you know what alignment this function guarantees?

Neither can I.
The latest documentation doesn't say anything about alignment of the memory returned by cuMemHostAlloc()/cuMemAllocHost().
While about cuMemAlloc()/cuMemAllocManaged() it explicitly says

The allocated memory is suitably aligned for any kind of variable.

Let me try some empirical tests and/or asking NVIDIA about it.

2. Shouldn't this assert be a runtime check instead?

Do you mean it should raise an exception rather than aborting ?

3. Is it OK to change alignment to 1 if it is 0?

I would take alignment == 0 as requiring no specific alignment, and just ignore it.
That is, the check could become

 assert((alignment == 0) or (reinterpret_cast<std::uintptr_t>(*result_ptr) % alignment == 0));

I agree with your interpretation of alignment == 0.

[jbrodman]

It would probably be good if we could through runtime errors instead of crashing.

Hmm.. CUDA doesn't seem to have user aligned allocations.

[romanovvlad]

@jbrodman Should we clarify what happens if required alignment is not supported by the implementation?

[romanovvlad]

@jinz2014 Could you please check if #1577 solves the issue?

[fwyzard]

Hmm.. CUDA doesn't seem to have user aligned allocations.

Rather, it seems that all allocations are aligned to 512 bytes.

[khaled-rahman]

Hi, I am also facing this problem. I compiled and ran a DPCPP code successfully in CPU. When, I compiled it for GPU it worked fine. However, when I offload the code to GPU, it shows a floating point exception (cored dumped). I was wondering whether the problem is resolved for this case.

[jeffhammond]

I too see this issue and wonder how I'm supposed to use USM on NVIDIA while this bug exists...

before alloc
---> piextUSMSharedAlloc(
       <unknown> : 0x7fffffffbae0
       <unknown> : 0xe9bcd0
       <unknown> : 0x7bd130
       <unknown> : 0
       <unknown> : 400000
       <unknown> : 0

Thread 1 "nstream-sycl-us" received signal SIGFPE, Arithmetic exception.
0x00007ffff65a46ab in cuda_piextUSMSharedAlloc () from /nfs/pdx/home/jrhammon/ISYCL/llvm/build/install/lib/libpi_cuda.so
(cuda-gdb) bt
#0  0x00007ffff65a46ab in cuda_piextUSMSharedAlloc () from /nfs/pdx/home/jrhammon/ISYCL/llvm/build/install/lib/libpi_cuda.so
#1  0x00007ffff7b48479 in cl::sycl::malloc_shared(unsigned long, cl::sycl::device const&, cl::sycl::context const&) ()
   from /nfs/pdx/home/jrhammon/ISYCL/llvm/build/install/lib/libsycl.so.2
#2  0x0000000000405b2a in float* cl::sycl::malloc_shared<float>(unsigned long, cl::sycl::queue const&) ()
#3  0x0000000000403e03 in void run<float>(cl::sycl::queue&, int, unsigned long) ()
#4  0x000000000040301d in main ()