absl/strings/internal/cordz_functions.cc

// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "absl/strings/internal/cordz_functions.h"

#include <atomic>
#include <cmath>
#include <limits>
#include <random>

#include "absl/base/attributes.h"
#include "absl/base/config.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/profiling/internal/exponential_biased.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {
namespace {

// The average interval until the next sample. A value of 0 disables profiling
// while a value of 1 will profile all Cords.
std::atomic<int> g_cordz_mean_interval(50000);

}  // namespace

#ifdef ABSL_INTERNAL_CORDZ_ENABLED

// Special negative 'not initialized' per thread value for cordz_next_sample.
static constexpr int64_t kInitCordzNextSample = -1;

ABSL_CONST_INIT thread_local SamplingState cordz_next_sample = {
    kInitCordzNextSample, 1};

// kIntervalIfDisabled is the number of profile-eligible events need to occur
// before the code will confirm that cordz is still disabled.
constexpr int64_t kIntervalIfDisabled = 1 << 16;

ABSL_ATTRIBUTE_NOINLINE int64_t
cordz_should_profile_slow(SamplingState& state) {

  thread_local absl::profiling_internal::ExponentialBiased
      exponential_biased_generator;
  int32_t mean_interval = get_cordz_mean_interval();

  // Check if we disabled profiling. If so, set the next sample to a "large"
  // number to minimize the overhead of the should_profile codepath.
  if (mean_interval <= 0) {
    state = {kIntervalIfDisabled, kIntervalIfDisabled};
    return 0;
  }

  // Check if we're always sampling.
  if (mean_interval == 1) {
    state = {1, 1};
    return 1;
  }

  if (cordz_next_sample.next_sample <= 0) {
    // If first check on current thread, check cordz_should_profile()
    // again using the created (initial) stride in cordz_next_sample.
    const bool initialized =
        cordz_next_sample.next_sample != kInitCordzNextSample;
    auto old_stride = state.sample_stride;
    auto stride = exponential_biased_generator.GetStride(mean_interval);
    state = {stride, stride};
    bool should_sample = initialized || cordz_should_profile() > 0;
    return should_sample ? old_stride : 0;
  }

  --state.next_sample;
  return 0;
}

void cordz_set_next_sample_for_testing(int64_t next_sample) {
  cordz_next_sample = {next_sample, next_sample};
}

#endif  // ABSL_INTERNAL_CORDZ_ENABLED

int32_t get_cordz_mean_interval() {
  return g_cordz_mean_interval.load(std::memory_order_acquire);
}

void set_cordz_mean_interval(int32_t mean_interval) {
  g_cordz_mean_interval.store(mean_interval, std::memory_order_release);
}

}  // namespace cord_internal
ABSL_NAMESPACE_END
}  // namespace absl