content/browser/histogram_synchronizer.h

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef CONTENT_BROWSER_HISTOGRAM_SYNCHRONIZER_H_
   6 #define CONTENT_BROWSER_HISTOGRAM_SYNCHRONIZER_H_
   7
   8 #include <string>
   9 #include <vector>
  10
  11 #include "base/basictypes.h"
  12 #include "base/callback.h"
  13 #include "base/memory/singleton.h"
  14 #include "base/synchronization/lock.h"
  15 #include "base/time/time.h"
  16 #include "content/browser/histogram_subscriber.h"
  17
  18 namespace base {
  19 class MessageLoop;
  20 }
  21
  22 namespace content {
  23
  24 // This class maintains state that is used to upload histogram data from the
  25 // various child processes, into the browser process. Such transactions are
  26 // usually instigated by the browser. In general, a child process will respond
  27 // by gathering snapshots of all internal histograms, calculating what has
  28 // changed since its last upload, and transmitting a pickled collection of
  29 // deltas.
  30 //
  31 // There are actually two modes of update request.  One is synchronous (and
  32 // blocks the UI thread, waiting to populate an about:histograms tab) and the
  33 // other is asynchronous, and used by the metrics services in preparation for a
  34 // log upload.
  35 //
  36 // To assure that all the processes have responded, a counter is maintained to
  37 // indicate the number of pending (not yet responsive) processes. To avoid
  38 // confusion about a response (i.e., is the process responding to a current
  39 // request for an update, or to an old request for an update) we tag each group
  40 // of requests with a sequence number. When an update arrives we can ignore it
  41 // (relative to the counter) if it does not relate to a current outstanding
  42 // sequence number.
  43 //
  44 // There is one final mode of use, where a renderer spontaneously decides to
  45 // transmit a collection of histogram data.  This is designed for use when the
  46 // renderer is terminating.  Unfortunately, renders may be terminated without
  47 // warning, and the best we can do is periodically acquire data from a tab, such
  48 // as when a page load has completed.  In this mode, the renderer uses a
  49 // reserved sequence number, different from any sequence number that might be
  50 // specified by a browser request.  Since this sequence number can't match an
  51 // outstanding sequence number, the pickled data is accepted into the browser,
  52 // but there is no impact on the counters.
  53
  54 class HistogramSynchronizer : public HistogramSubscriber {
  55  public:
  56   enum ProcessHistogramRequester {
  57     UNKNOWN,
  58     ASYNC_HISTOGRAMS,
  59   };
  60
  61   // Return pointer to the singleton instance for the current process, or NULL
  62   // if none.
  63   static HistogramSynchronizer* GetInstance();
  64
  65   // Contact all processes, and get them to upload to the browser any/all
  66   // changes to histograms. This method is called from about:histograms.
  67   static void FetchHistograms();
  68
  69   // Contact all child processes, and get them to upload to the browser any/all
  70   // changes to histograms.  When all changes have been acquired, or when the
  71   // wait time expires (whichever is sooner), post the callback to the
  72   // specified message loop. Note the callback is posted exactly once.
  73   static void FetchHistogramsAsynchronously(base::MessageLoop* callback_thread,
  74                                             const base::Closure& callback,
  75                                             base::TimeDelta wait_time);
  76
  77  private:
  78   friend struct DefaultSingletonTraits<HistogramSynchronizer>;
  79
  80   class RequestContext;
  81
  82   HistogramSynchronizer();
  83   ~HistogramSynchronizer() override;
  84
  85   // Establish a new sequence number, and use it to notify all processes
  86   // (renderers, plugins, GPU, etc) of the need to supply, to the browser,
  87   // any/all changes to their histograms. |wait_time| specifies the amount of
  88   // time to wait before cancelling the requests for non-responsive processes.
  89   void RegisterAndNotifyAllProcesses(ProcessHistogramRequester requester,
  90                                      base::TimeDelta wait_time);
  91
  92   // -------------------------------------------------------
  93   // HistogramSubscriber methods for browser child processes
  94   // -------------------------------------------------------
  95
  96   // Update the number of pending processes for the given |sequence_number|.
  97   // This is called on UI thread.
  98   void OnPendingProcesses(int sequence_number,
  99                           int pending_processes,
 100                           bool end) override;
 101
 102   // Send histogram_data back to caller and also record that we are waiting
 103   // for one less histogram data from child process for the given sequence
 104   // number. This method is accessible on UI thread.
 105   void OnHistogramDataCollected(
 106       int sequence_number,
 107       const std::vector<std::string>& pickled_histograms) override;
 108
 109   // Set the callback_thread_ and callback_ members. If these members already
 110   // had values, then as a side effect, post the old callback_ to the old
 111   // callaback_thread_.  This side effect should not generally happen, but is in
 112   // place to assure correctness (that any tasks that were set, are eventually
 113   // called, and never merely discarded).
 114   void SetCallbackTaskAndThread(base::MessageLoop* callback_thread,
 115                                 const base::Closure& callback);
 116
 117   void ForceHistogramSynchronizationDoneCallback(int sequence_number);
 118
 119   // Internal helper function, to post task, and record callback stats.
 120   void InternalPostTask(base::MessageLoop* thread,
 121                         const base::Closure& callback);
 122
 123   // Gets a new sequence number to be sent to processes from browser process.
 124   int GetNextAvailableSequenceNumber(ProcessHistogramRequester requester);
 125
 126   // This lock_ protects access to all members.
 127   base::Lock lock_;
 128
 129   // When a request is made to asynchronously update the histograms, we store
 130   // the task and thread we use to post a completion notification in
 131   // callback_ and callback_thread_.
 132   base::Closure callback_;
 133   base::MessageLoop* callback_thread_;
 134
 135   // We don't track the actual processes that are contacted for an update, only
 136   // the count of the number of processes, and we can sometimes time-out and
 137   // give up on a "slow to respond" process.  We use a sequence_number to be
 138   // sure a response from a process is associated with the current round of
 139   // requests (and not merely a VERY belated prior response).
 140   // All sequence numbers used are non-negative.
 141   // last_used_sequence_number_ is the most recently used number (used to avoid
 142   // reuse for a long time).
 143   int last_used_sequence_number_;
 144
 145   // The sequence number used by the most recent asynchronous update request to
 146   // contact all processes.
 147   int async_sequence_number_;
 148
 149   DISALLOW_COPY_AND_ASSIGN(HistogramSynchronizer);
 150 };
 151
 152 }  // namespace content
 153
 154 #endif  // CONTENT_BROWSER_HISTOGRAM_SYNCHRONIZER_H_