Add tagged histogram with reasons of purging the dom storage databases

content/browser/dom_storage/dom_storage_context_impl.cc

 // Copyright (c) 2012 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 "content/browser/dom_storage/dom_storage_context_impl.h"
 
 #include <stddef.h>
 #include <stdlib.h>
 
 #include "base/bind.h"
@@ skipping 421 matching lines @@
     return;
 
   DOMStorageNamespace::UsageStatistics initial_stats =
       GetTotalNamespaceStatistics(namespaces_);
 
   // Track the total localStorage cache size.
   UMA_HISTOGRAM_CUSTOM_COUNTS("LocalStorage.BrowserLocalStorageCacheSizeInKB",
                               initial_stats.total_cache_size / 1024, 1, 100000,
                               50);
 
+  const char* purge_reason = nullptr;
   if (purge_option == PURGE_IF_NEEDED) {
     // Purging is done based on the cache sizes without including the database
     // size since it can be expensive trying to estimate the sqlite usage for
     // all databases. For low end devices purge all inactive areas.
-    bool should_purge =
-        initial_stats.inactive_area_count &&
-        (is_low_end_device_ || initial_stats.total_cache_size > kMaxCacheSize ||
-         initial_stats.total_area_count > kMaxStorageAreaCount);
-    if (!should_purge)
+    if (!initial_stats.inactive_area_count)
+      return;
+    if (is_low_end_device_)
+      purge_reason = "InactiveOnLowEndDevice";
+    else if (initial_stats.total_cache_size > kMaxCacheSize)
+      purge_reason = "SizeLimitExceeded";
+    else if (initial_stats.total_area_count > kMaxStorageAreaCount)
+      purge_reason = "AreaCountLimitExceeded";
+    if (!purge_reason)
       return;
 
     purge_option = PURGE_UNOPENED;
   }
 
   bool aggressively = purge_option == PURGE_AGGRESSIVE;
   for (const auto& it : namespaces_)
     it.second->PurgeMemory(aggressively);
 
   // Track the size of cache purged.
-  UMA_HISTOGRAM_CUSTOM_COUNTS(
-      "LocalStorage.BrowserLocalStorageCachePurgedInKB",
+  size_t purged_size_kib =
       (initial_stats.total_cache_size -
        GetTotalNamespaceStatistics(namespaces_).total_cache_size) /
-          1024,
-      1, 100000, 50);
+      1024;
+  std::string full_histogram_name =
+      std::string("LocalStorage.BrowserLocalStorageCachePurgedInKB.") +

[Maria, 2016/06/16 00:08:15]

you also need to update histograms.xml file with the new names.

[ssid, 2016/06/16 01:01:24]

I copied the pattern from "Sqlite.SizeKB" it also seems to add multiple
histograms with a "." after the name similar to what i have done here.
The dashboard adds the existing description with the last part of name. Like
https://uma.googleplex.com/p/chrome/histograms/?endDate=06-14-2016&dayCount=1...

+      purge_reason;
+  base::HistogramBase* histogram = base::Histogram::FactoryGet(
+      full_histogram_name, 1, 100000, 50,
+      base::HistogramBase::kUmaTargetedHistogramFlag);
+  if (histogram)
+    histogram->Add(purged_size_kib);
+  UMA_HISTOGRAM_CUSTOM_COUNTS("LocalStorage.BrowserLocalStorageCachePurgedInKB",
+                              purged_size_kib, 1, 100000, 50);
 }
 
 bool DOMStorageContextImpl::OnMemoryDump(
     const base::trace_event::MemoryDumpArgs& args,
     base::trace_event::ProcessMemoryDump* pmd) {
   if (session_storage_database_)
     session_storage_database_->OnMemoryDump(pmd);
   if (args.level_of_detail ==
       base::trace_event::MemoryDumpLevelOfDetail::BACKGROUND) {
     DOMStorageNamespace::UsageStatistics total_stats =
@@ skipping 80 matching lines @@
   if (!deletable_persistent_namespace_ids_.empty()) {
     task_runner_->PostDelayedTask(
         FROM_HERE, base::Bind(
             &DOMStorageContextImpl::DeleteNextUnusedNamespace,
             this),
         base::TimeDelta::FromSeconds(kSessionStoraceScavengingSeconds));
   }
 }
 
 }  // namespace content