rocm-systems/samples/pcsampler/code_printing_sample

ROCProfiler PC sampling example code

The ROCProfiler library includes an API to enable periodic sampling of the GPU program counter during kernel execution. This program demonstrates the PC sampling API, with additional code to illustrate a typical non-trivial use case: correlation of sampled PC addresses with their disassembled machine code, as well as source code and symbolic debugging information if available.

Building the demo program

If your ROCm installation already includes ROCProfiler, the only requirements to build the demo program are:

• GNU make
• libdw (not libdwarf)
• libelf

If ROCm is installed in the standard location (/opt/rocm), running make in the same directory as this README should work; otherwise, set ROCM_PATH to the location of the ROCm installation in your environment and ROCPROFILER_PATH to the location of the ROCProfiler source repo before running make.

If your ROCm installation does not include ROCProfiler, you will need to build it yourself. This demo program will be built as part of that process. See the main ROCProfiler README for additional requirements and directions.

Running the demo program

The demo program simply fills a vector with random 64-bit unsigned integers and tallies the count of those greater than the mandatory MIN argument:

usage: code_printing_sample [OPTION]... MIN [SEED]
  -d DEV        HIP device number
  -n LEN        Length of random integer array
  -D            Print kernel disassembly
  -P            Print source and disassembly of sampled PC locations
where
  DEV : i32
  MIN : u64
  LEN : u64
  SEED : u64

Defaults and troubleshooting

• -d: use HIP device 0
• -n: 4194304 (1024 * 1024 * 4)
• -D: false
• -P: false
• SEED: random seed; taken from the system's monotonic clock

The program contains two trivial GPU kernels: an implementation of memset, and the parallel counting procedure. Because the actual point is to demonstrate the PC sampling functionality, it is recommended to use the -n option with an argument such that the allocated vector fits in the smaller of available host as well as device memory, but sufficiently large argument such that the kernels run long enough for ROCProfiler to actually collect some samples.

In order for the -P option to display source, the demo program must have been built with debug symbols (at least -gdwarf-4). Any optimization level is fine, but if the kernels run too quickly for ROCProfiler to collect any samples even when a very large vector is given with the -n option, try rebuilding the demo program without optimizations by adding -O0 to the hipcc compilation flags.

Files

• main.cpp: initializes ROCProfiler and PC sampling and runs the GPU kernels
• code_printing.cpp: inspects the ELF and DWARF info for the GPU programs embedded in the host binary and uses amd-dbgapi to print disassembly and source
• disassembly.cpp: wrapper for code_printing.cpp

PC sampling API

Adding PC sampling to a program already using the ROCProfiler API requires only two changes:

1. Call rocprofiler_create_filter to create a ROCPROFILER_PC_SAMPLING_COLLECTION filter, then rocprofiler_set_filter_buffer to add the filter to the desired buffer (see functions main and run_kernel in main.cpp)

2. Handle records of kind ROCPROFILER_PC_SAMPLING_RECORD in the buffer callback function. These should be cast to rocprofiler_record_pc_sample_t * (see function callback_flush_fn in main.cpp)

Like all ROCProfiler records, PC sample records contain a standard header followed by one or more payloads:

/**
 * PC sample record: contains the program counter/instruction pointer observed
 * during periodic sampling of a kernel
 */
typedef struct {
  /**
   * ROCProfiler General Record base header to identify the id and kind of every
   * record
   */
  rocprofiler_record_header_t header;
  /**
   * PC sample data
   */
  rocprofiler_pc_sample_t pc_sample;
} rocprofiler_record_pc_sample_t;

PC samples are delivered via the normal ROCProfiler buffer callback mechanism, along with some additional information allowing each sample to be associated with a unique, individual kernel execution:

/**
 * An individual PC sample
 */
typedef struct {
  /**
   * Kernel dispatch ID.  This is used by PC sampling to associate samples with
   * individual dispatches and is unrelated to any user-supplied correlation ID
   */
  rocprofiler_kernel_dispatch_id_t dispatch_id;
  union {
    /**
     * Host timestamp
     */
    rocprofiler_timestamp_t timestamp;
    /**
     * GPU clock counter (not currently used)
     */
    uint64_t cycle;
  };
  /**
   * Sampled program counter
   */
  uint64_t pc;
  /**
   * Sampled shader element
   */
  uint32_t se;
  /**
   * Sampled GPU agent
   */
  rocprofiler_agent_id_t gpu_id;
} rocprofiler_pc_sample_t;

PC sampling is started and stopped with rocprofiler_start_session and rocprofiler_terminate_session, just like other profiling activities.