// Copyright 2017 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "third_party/blink/public/common/experiments/memory_ablation_experiment.h"

#include <algorithm>
#include <limits>

#include "base/bind.h"
#include "base/bind_helpers.h"
#include "base/debug/alias.h"
#include "base/metrics/field_trial_params.h"
#include "base/process/process_metrics.h"
#include "base/rand_util.h"
#include "base/sequenced_task_runner.h"
#include "base/system/sys_info.h"

namespace blink {

const base::Feature kMemoryAblationFeature{"MemoryAblation",
                                           base::FEATURE_DISABLED_BY_DEFAULT};
const base::Feature kRendererMemoryAblationFeature{
    "RendererMemoryAblation", base::FEATURE_DISABLED_BY_DEFAULT};

const char kMemoryAblationFeatureSizeParam[] = "Size";
const char kMemoryAblationFeatureMinRAMParam[] = "MinRAM";
const char kMemoryAblationFeatureMaxRAMParam[] = "MaxRAM";

// Since MaybeStart() is called during startup, we delay the allocation
// to avoid slowing things down.
constexpr size_t kAllocationDelaySeconds = 5;

// We need to touch allocated memory to "dirty" it. However, touching
// large chunks of memory (even a single byte per page) can be costly
// (~60ms per 10MiB on Nexus 5). So we touch in chunks to allow other
// things to happen in between. With the following values we'll touch
// 2MiB per second, spending ~10% of 250ms time slice per chunk.
constexpr size_t kTouchDelayMilliseconds = 250;
constexpr size_t kTouchChunkSize = 512 * 1024;

MemoryAblationExperiment::MemoryAblationExperiment() {}

MemoryAblationExperiment::~MemoryAblationExperiment() {}

MemoryAblationExperiment* MemoryAblationExperiment::GetInstance() {
  static auto* instance = new MemoryAblationExperiment();
  return instance;
}

void MemoryAblationExperiment::MaybeStart(
    scoped_refptr<base::SequencedTaskRunner> task_runner) {
  MaybeStartInternal(kMemoryAblationFeature, task_runner);
}

void MemoryAblationExperiment::MaybeStartForRenderer(
    scoped_refptr<base::SequencedTaskRunner> task_runner) {
  MaybeStartInternal(kRendererMemoryAblationFeature, task_runner);
}

void MemoryAblationExperiment::MaybeStartInternal(
    const base::Feature& memory_ablation_feature,
    scoped_refptr<base::SequencedTaskRunner> task_runner) {
  int min_ram_mib = base::GetFieldTrialParamByFeatureAsInt(
      memory_ablation_feature, kMemoryAblationFeatureMinRAMParam,
      0 /* default value */);
  int max_ram_mib = base::GetFieldTrialParamByFeatureAsInt(
      memory_ablation_feature, kMemoryAblationFeatureMaxRAMParam,
      std::numeric_limits<int>::max() /* default value */);
  if (base::SysInfo::AmountOfPhysicalMemoryMB() > max_ram_mib ||
      base::SysInfo::AmountOfPhysicalMemoryMB() <= min_ram_mib) {
    return;
  }

  int size = base::GetFieldTrialParamByFeatureAsInt(
      memory_ablation_feature, kMemoryAblationFeatureSizeParam,
      0 /* default value */);
  if (size > 0) {
    GetInstance()->Start(task_runner, size);
  }
}

void MemoryAblationExperiment::Start(
    scoped_refptr<base::SequencedTaskRunner> task_runner,
    size_t memory_size) {
  DCHECK(!task_runner_) << "Already started";
  task_runner_ = task_runner;
  // This class is a singleton, so "Unretained(this)" below is fine.
  task_runner_->PostDelayedTask(
      FROM_HERE,
      base::BindOnce(&MemoryAblationExperiment::AllocateMemory,
                     base::Unretained(this), memory_size),
      base::TimeDelta::FromSeconds(kAllocationDelaySeconds));
}

void MemoryAblationExperiment::AllocateMemory(size_t size) {
  memory_size_ = size;
  memory_.reset(new uint8_t[size]);
  ScheduleTouchMemory(0);
}

void MemoryAblationExperiment::TouchMemory(size_t offset) {
  if (memory_) {
    size_t page_size = base::GetPageSize();
    uint8_t* memory = memory_.get();
    size_t max_offset = std::min(offset + kTouchChunkSize, memory_size_);
    if (offset == 0) {
      // Fill the first page with random bytes. We'll use this page as
      // a template for all other pages.
      size_t rand_size = std::min(page_size, memory_size_);
      base::RandBytes(memory, rand_size);
      offset += rand_size;
    }
    for (; offset < max_offset; offset += page_size) {
      // Copy from the first page.
      size_t copy_size = std::min(page_size, max_offset - offset);
      memcpy(memory + offset, memory, copy_size);
      // Make this page different by stamping it with the offset.
      if (copy_size >= sizeof(size_t)) {
        *reinterpret_cast<size_t*>(memory + offset) = offset;
      }
    }
    // Make sure compiler doesn't optimize away the writes.
    base::debug::Alias(memory);
    if (offset < memory_size_) {
      ScheduleTouchMemory(offset);
    }
  }
}

void MemoryAblationExperiment::ScheduleTouchMemory(size_t offset) {
  // This class is a singleton, so "Unretained(this)" below is fine.
  task_runner_->PostDelayedTask(
      FROM_HERE,
      base::BindOnce(&MemoryAblationExperiment::TouchMemory,
                     base::Unretained(this), offset),
      base::TimeDelta::FromMilliseconds(kTouchDelayMilliseconds));
}

}  // namespace blink