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PerfMonReg.md

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Metrics Collection with Performance Monitoring (PM) Register

Overview

Performance monitoring (PM) register is a special register inside Intel(R) Processor Graphics execution unit (EU) that can be set to collect some particular hardware events (one event at a time).

To set collection for an event, one have to define three register fields:

  • Event ID - which event to collect;
  • Coarse Filter - filter by shader type and EU raws;
  • Fine Filter - filter by instruction type and EUs inside a raw.

Exact event list and filter options can be found in Programmer's Reference Manual for Intel(R) Processor Graphics, Volume 14: Observability.

Supported OS:

  • Linux

Supported HW:

  • Intel(R) Processor Graphics GEN9+

Needed Headers:

Needed Libraries:

How To Use

Event Set

To tune PM register to collect a particular event one may use Direct Rendering Manager (DRM) interface to deal with Intel(R) Processor Graphics driver.

First of all one have to open DRM interface for i915 driver:

int fd = drmOpenWithType("i915", NULL, DRM_NODE_RENDER);
if (fd < 0) {
  fd = drmOpenWithType("i915", NULL, DRM_NODE_PRIMARY);
}
assert(fd >= 0);

Second step is to prepare PM register configuration:

#define PERF_GUID_LENGTH 37

// PM register is in a group of flexible registers
std::vector<unsigned> flex_regs(2);
flex_regs[0] = PERF_REG_OFFSET;
flex_regs[1] = reg_value; // combined filters + event_id value

std::string guid = GetGuid(flex_regs);

struct drm_i915_perf_oa_config param = {0,};
assert(sizeof(param.uuid) == PERF_GUID_LENGTH - 1);
assert(flex_regs.size() % 2 == 0);

memcpy(param.uuid, guid.c_str(), sizeof(param.uuid));

// Register values
param.boolean_regs_ptr = 0;
param.mux_regs_ptr = 0;
param.flex_regs_ptr = reinterpret_cast<uint64_t>(flex_regs.data());

// Number of register values
param.n_boolean_regs = 0;
param.n_mux_regs = 0;
param.n_flex_regs = static_cast<uint32_t>(flex_regs.size() / 2);

config_id = SendIoctl(fd, DRM_IOCTL_I915_PERF_ADD_CONFIG, &param);

The only unobvious field here is GUID, it should be formed as following:

std::string GetGuid(const std::vector<unsigned>& regs) {
  std::string regs_string;
  for (size_t i = 0; i < regs.size(); ++i) {
    regs_string += regs[i];
  }

  char guid[PERF_GUID_LENGTH] = { 0 };
  std::hash<std::string> hash;
  snprintf(guid, sizeof(guid), "%08x-%04x-%04x-%04x-%012x", 0, 0, 0, 0,
    static_cast<uint32_t>(hash(regs_string)));
  return guid;
}

Finally one have to use received config_id to actually enable event collection:

uint64_t properties[] = {
  DRM_I915_PERF_PROP_SAMPLE_OA, 1,
  DRM_I915_PERF_PROP_OA_METRICS_SET, static_cast<uint64_t>(config_id),
  DRM_I915_PERF_PROP_OA_FORMAT, I915_OA_FORMAT_A32u40_A4u32_B8_C8 };

struct drm_i915_perf_open_param param = { 0, };
param.flags = 0;
param.flags |= I915_PERF_FLAG_FD_CLOEXEC;
param.flags |= I915_PERF_FLAG_FD_NONBLOCK;
param.properties_ptr = reinterpret_cast<uint64_t>(properties);
param.num_properties = sizeof(properties) / (2 * sizeof(uint64_t));

int stream_fd = SendIoctl(fd, DRM_IOCTL_I915_PERF_OPEN, &param);

Starting from this moment PM register will be incremented every cycle target event happened.

To stop event collection and release stream configuration one have to close stream descriptor:

drmClose(stream_fd);
drmClose(fd);

Event Read

Since PM register is accessible only from GPU kernel, it's impossible to read it directly from host side. One of the possible ways to read PM results is to use a special Graphics Technology Pin (GT Pin) probe while binary instrumentation called perfmon.

Refer to Graphics Technology Pin (GT Pin) chapter to learn more on binary instrumentation techniques.

Usage Details

  • refer to Programmer's Reference Manual for Intel(R) Processor Graphics, Volume 14: Observability to learn more on performance monitoring (PM) register
  • refer to Graphics Technology Pin (GT Pin) Binary Instrumentation chapter to learn more on the ways of reading PM events

Samples