Hi all,
We are using the recently uploaded gpu-compute patches to run some OpenCL
2.0 benchmarks in AMD SDK.
When we run the SVMAtomicBinaryTreeInsert benchmark on the simulator, we
run into an unexpected termination:
gem5.opt: build/HSAIL_X86/mem/ruby/system/GPUCoalescer.cc:310: void
GPUCoalescer::insertKernel(int, PacketPtr): Assertion
`kernelEndList.count(wavefront_id) == 0' failed.
Program aborted at tick 366329000
Looking closer at the simulator, we saw that when a GPUCoalescer tried to
handle a MemFenceReq and the execution was being in the middle of an
outstanding release (VIPERCoalescer::makeRequest() - line 99), it called
the GPUCoalescer::insertKernel (line 112) that tried but failed to
assert "kernelEndList.count(wavefront_id)
== 0". A code comment in GPUCoalescer::insertKernel also mentions that the
simulator could possibly hang in the future if the assertion is not done.
Looking at the kernel and also its assembly code, we suspect that a
memfence instruction placed near the end of the kernel caused the problem.
// A snippet of source code
do {
...
if (exFlag){
if (child) {
...
} else {
...
done = 1;
}
atomic_compare_exchange_strong()
}
atomic_work_item_mem_fence()
} while (!done);
return;
Please find both kernel file and its assembly code in the attachment. Our
running configuration is:
- We used apu_se.py configuration and VIPER protocol and set num_CUs to 8
- The Tree Insert programs adds 256 new nodes to the initial tree. No node
is added by CPU.
We think the above do-while loop makes some threads execute the kernel end
earlier than others in a wavefront. When the coalescer executes the
memfence and checks the list of kernel end, it may see a kernel end request
already existing for the wavefront and the assertion fails.
Is there any one else running into the same problem? If so, could you give
us a hint on what caused the problem and how to fix it?
Thank you!
Regards,
Tuan Ta
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