base: Add blame context

base/debug/blame_context.h

Index: base/debug/blame_context.h
diff --git a/base/debug/blame_context.h b/base/debug/blame_context.h
new file mode 100644
index 0000000000000000000000000000000000000000..9e54ce0c96051f76760271b70cba64dc390224d4
--- /dev/null
+++ b/base/debug/blame_context.h
@@ -0,0 +1,126 @@
+// Copyright 2016 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.
+
+#ifndef BASE_DEBUG_BLAME_CONTEXT_H_
+#define BASE_DEBUG_BLAME_CONTEXT_H_
+
+#include <inttypes.h>
+
+#include "base/base_export.h"
+#include "base/macros.h"
+#include "base/trace_event/trace_event.h"
+
+namespace base {
+namespace trace_event {
+class TracedValue;
+}
+
+namespace debug {
+
+// Abstract blame context base class which can be used for attributing work.
+class BASE_EXPORT BlameContextBase {
+ public:
+  // Indicate that the current thread is now doing work which should count
+  // against this blame context.
+  virtual void Enter() = 0;
+
+  // Leave and stop doing work for a previously entered blame context. If
+  // another blame context of the same type was entered prior to this one, it
+  // becomes the active blame context for this thread again.
+  virtual void Leave() = 0;
+
+ protected:
+  BlameContextBase() {}
+  virtual ~BlameContextBase() {}
+
+  DISALLOW_COPY_AND_ASSIGN(BlameContextBase);
+};
+
+// A blame context represents a logical unit to which we want to attribute
+// different costs (e.g., CPU, network, or memory usage). An example of a blame
+// context is an <iframe> element on a web page. Different subsystems can
+// "enter" and "leave" blame contexts to indicate that they are doing work which
+// should be accounted against this blame context.
+//
+// Each blame context can optionally have a parent context, forming a blame
+// context tree. When work is attributed to a particular blame context, it
+// is considered to count against all of that context's children too. This is

[chiniforooshan, 2016/03/10 16:18:55]

Oh, does this attributing work to all children work for the corss-process
solution? I just added a comment in the design doc, too: if we are doing some
work in process A in frame context AF and then delegate some of the work to two
different processes B and C. They will create their own contexts BF and CF,
respectively (since contexts are per-process things). What should be the
parent-child relationships so that the work attribution work correctly?

[Sami, 2016/03/10 17:59:02]

I commented on the doc but I'll repeat it here too: the relationship between
frame contexts across processes isn't really parent-child but more like a
sibling link that helps us connect the different shards of a single frame
together.

In your example the browser frame tree would have nodes AF', BF' and CF' with
links to the respective blame contexts in the three renderers. When it's time to
find out how much work AF', BF' and CF' are doing, we follow the links to each
respective blame context and sum everything up, obeying the parent-child
relationships that form the three separate per-renderer frame trees.

Does that make sense?

+// useful when work cannot be exactly attributed into a more specific context.
+// For example, Javascript garbage collection generally needs to inspect all
+// objects on a page instead looking at each <iframe> individually. In this case
+// the work should be attributed to a blame context which is the parent of all
+// <iframe> blame contexts.
+class BASE_EXPORT BlameContext
+    : public BlameContextBase,
+      public trace_event::TraceLog::EnabledStateObserver {
+ public:
+  // Construct a blame context of type |name|, belonging to the tracing category
+  // |category|, identified by |id| from to the |scope| namespace. This blame
+  // context will not have a parent. Note that all strings must have application
+  // lifetime.
+  BlameContext(const char* category,
+               const char* name,
+               const char* scope,
+               int64_t id);
+
+  // Construct a blame context of type |name|, belonging to the tracing category
+  // |category|, identified by |id| from to the |scope| namespace.
+  // |parent_context| identifies the parent for this blame context. It must
+  // belong to the same blame context tree as this context, i.e., |name| must
+  // equal |parent_context.name()|. Note that all strings must have application
+  // lifetime.
+  BlameContext(const char* category,
+               const char* name,
+               const char* scope,
+               int64_t id,
+               const BlameContext& parent_context);
+
+  // BlameContextBase implementation:
+  void Enter() override;
+  void Leave() override;
+
+  // Record a snapshot of the blame context. This is normally only needed if a
+  // blame context subclass defines custom properties (see AsValueInto) and one
+  // or more of those properties have changed.
+  void TakeSnapshot();
+
+  const char* category() const { return category_; }
+  const char* name() const { return name_; }
+  const char* scope() const { return scope_; }
+  int64_t id() const { return id_; }
+
+  // trace_event::TraceLog::EnabledStateObserver implementation:
+  void OnTraceLogEnabled() override;
+  void OnTraceLogDisabled() override;
+
+ protected:
+  ~BlameContext() override;
+
+  // Serialize the properties of this blame context into |state|. Subclasses can
+  // override this method to record additional properties (e.g, the URL for an
+  // <iframe> blame context). Note that an overridden implementation must still
+  // call this base method.
+  virtual void AsValueInto(trace_event::TracedValue* state);
+
+ private:
+  void Initialize();
+
+  // The following string pointers have application lifetime.
+  const char* category_;
+  const char* name_;
+  const char* scope_;
+  const int64_t id_;
+
+  const char* parent_scope_;
+  const int64_t parent_id_;
+
+  const unsigned char* category_group_enabled_;
+
+  DISALLOW_COPY_AND_ASSIGN(BlameContext);
+};
+
+}  // namespace debug
+}  // namespace base
+
+#endif  // BASE_DEBUG_BLAME_CONTEXT_H_