abseil-cpp/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/exponential_biased.h"
#include "absl/base/internal/raw_logging.h"

// TODO(b/162942788): weak 'cordz_disabled' value.
// A strong version is in the 'cordz_disabled_hack_for_odr' library which can
// be linked in to disable cordz at compile time.
extern "C" {
bool absl_internal_cordz_disabled ABSL_ATTRIBUTE_WEAK = false;
}

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

ABSL_CONST_INIT thread_local int64_t cordz_next_sample = 0;

// 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 bool cordz_should_profile_slow() {
  // TODO(b/162942788): check if profiling is disabled at compile time.
  if (absl_internal_cordz_disabled) {
    ABSL_RAW_LOG(WARNING, "Cordz info disabled at compile time");
    // We are permanently disabled: set counter to highest possible value.
    cordz_next_sample = std::numeric_limits<int64_t>::max();
    return false;
  }

  thread_local absl::base_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) {
    cordz_next_sample = kIntervalIfDisabled;
    return false;
  }

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

  if (cordz_next_sample <= 0) {
    cordz_next_sample = exponential_biased_generator.GetStride(mean_interval);
    return true;
  }

  --cordz_next_sample;
  return false;
}

void cordz_set_next_sample_for_testing(int64_t next_sample) {
  cordz_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