[Hacky prototype] Create a shared-memory high-performance reporting service.

services/blamer/shared_memory_heap.cc

Index: services/blamer/shared_memory_heap.cc
diff --git a/services/blamer/shared_memory_heap.cc b/services/blamer/shared_memory_heap.cc
new file mode 100644
index 0000000000000000000000000000000000000000..35fb099c3e66d6a0603114acbd8fc1a23d64df13
--- /dev/null
+++ b/services/blamer/shared_memory_heap.cc
@@ -0,0 +1,143 @@
+// 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 "services/blamer/shared_memory_heap.h"
+
+#include "services/blamer/public/interfaces/shared_memory_heap_registry.mojom.h"
+
+namespace blamer {
+
+namespace {
+
+// A sentinel value that indicates a process or thread is currently allocating
+// a new slab.
+// DO NOT SUBMIT
+base::PersistentMemoryAllocator* kCurrentSlabLockSentinel =
+    reinterpret_cast<base::PersistentMemoryAllocator*>(1);
+
+}  // namespace
+
+SharedMemoryHeap::SharedMemoryHeap(
+    mojom::SharedMemoryHeapRegistryPtr heap_registry)
+    : heap_registry_(std::move(heap_registry)),
+      current_slab_(nullptr) {
+}
+
+SharedMemoryHeap::~SharedMemoryHeap() = default;
+
+base::PersistentMemoryAllocator* SharedMemoryHeap::GetCurrentSlab() {
+  while (true) {
+    auto* slab = current_slab_.load();
+    // If the sentinel value is encountered another thread is currently creating
+    // a slab. Busy loop until it's ready.
+    if (slab == kCurrentSlabLockSentinel)
+      continue;
+    // If a nullptr is encountered then this is racing to be the first thread to
+    // create a new slab.
+    if (slab == nullptr)
+      return CreateNewSlab(slab);
+    // Otherwise, a valid slab has been created and is in use.
+    return slab;
+  }
+}
+
+base::PersistentMemoryAllocator* SharedMemoryHeap::CreateNewSlab(
+    base::PersistentMemoryAllocator* current_slab) {
+  // Only one thread gets the opportunity to create a new slab. That is the
+  // thread that successfully sets |current_slab_| to the sentinel value.
+  if (!current_slab_.compare_exchange_strong(current_slab,
+                                             kCurrentSlabLockSentinel)) {
+
+    // If another thread beat us here then wait until they've created a new
+    // allocator.
+    while (true) {
+      auto* slab = current_slab_.load();
+      if (slab != kCurrentSlabLockSentinel && slab != current_slab)
+        return slab;
+    }
+  }
+
+  // Only the thread that acquired the right to create a new slab makes it here.
+
+  base::PersistentMemoryAllocator* new_slab = nullptr;
+  {
+    // First create and map the slab of shared memory.
+    mojo::ScopedSharedBufferHandle buffer =
+        mojo::SharedBufferHandle::Create(kSlabSize);
+    mojo::ScopedSharedBufferMapping mapping = buffer->Map(kSlabSize);
+
+    // Build a persistent allocator over it.
+    size_t page_size = 0;
+    size_t slab_id = slabs_.size();
+    std::unique_ptr<base::PersistentMemoryAllocator> allocator =
+        base::MakeUnique<base::PersistentMemoryAllocator>(
+            mapping.get(), kSlabSize, page_size, slab_id,
+            "SharedMemoryAllocatorSlab", false);
+    new_slab = allocator.get();
+
+    // Add this to the list of slabs.
+    Slab slab;
+    slab.buffer = std::move(buffer);
+    slab.mapping = std::move(mapping);
+    slab.allocator = std::move(allocator);
+    slabs_.push_back(std::move(slab));

[Sami, 2017/05/19 13:44:04]

I wondering about re-entrancy issues here, e.g., this could end up going into
memory-infra which might call back into CreateNewSlab. Maybe we should at least
add some DCHECKs against that?

[chrisha, 2017/05/25 18:05:07]

Yeah, fair point. Depending on what the memory-infra code itself is doing we
might actually need to explicitly handle this case rather than just DCHECK
against it.

+
+    // Register the new slab with the central service registry.
+    auto& local_slab = slabs_.back();
+    mojom::SharedMemoryHeapSlab shared_slab;
+    shared_slab.buffer = local_slab.buffer->Clone();
+    shared_slab.size = kSlabSize;
+    shared_slab.id = slab_id;
+
+  }
+
+  // Update the current slab. This will unblock other threads waiting for a new
+  // slab.
+  base::PersistentMemoryAllocator* expected = kCurrentSlabLockSentinel;
+  CHECK(current_slab_.compare_exchange_strong(expected, new_slab));
+
+  return new_slab;
+}
+
+SharedMemoryHeap::LocalPointer SharedMemoryHeap::Allocate(
+    HeapObjectType object_type, size_t object_size) {
+  auto* slab = GetCurrentSlab();
+
+  // Allocate the object. If it fails get a new slab and try again.
+  // TODO(chrisha): Clean up the failure path that causes slabs to be allocated
+  // until memory is exhausted!
+  uint32_t ref = slab->Allocate(object_size, object_type);
+  if (ref == 0) {
+    slab = CreateNewSlab(slab);
+    ref = slab->Allocate(object_size, object_type);
+  }

[Sami, 2017/05/19 13:44:04]

CHECK(ref); ?

+
+  LocalPointer local_pointer = {};
+  local_pointer.slab = slab;
+  local_pointer.pointer.slab_id = slab->Id();
+  local_pointer.pointer.slab_offset = ref;
+  return local_pointer;
+}
+
+void SharedMemoryHeap::Free(HeapObjectType object_type,
+                            const LocalPointer& pointer) {
+  // TODO(chrisha): Create free lists, and repurpose this memory! For now,
+  // simply mark it as FREED and abandon it.
+  auto* slab = reinterpret_cast<base::PersistentMemoryAllocator*>(
+      pointer.slab);
+  uint32_t ref = pointer.pointer.slab_offset;
+  slab->ChangeType(ref, HeapObjectType::FREED, object_type, false);
+}
+
+SharedMemoryHeap::Slab::Slab() {}
+
+SharedMemoryHeap::Slab::Slab(Slab&& other)
+    : buffer(std::move(other.buffer)),
+      mapping(std::move(other.mapping)),
+      allocator(std::move(other.allocator)) {
+}
+
+SharedMemoryHeap::Slab::~Slab() {}
+
+}  // namespace blamer