Add histograms to track which histograms are going negative and why.

base/metrics/persistent_sample_map.cc

 // 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.
 
 #include "base/metrics/persistent_sample_map.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/persistent_histogram_allocator.h"
@@ skipping 121 matching lines @@
       reason = PERSISTENT_SPARSE_ACCUMULATE_WENT_NEGATIVE;
     *local_count_ptr += count;
   } else {
     *local_count_ptr += count;
     if (*local_count_ptr < 0)
       reason = PERSISTENT_SPARSE_ACCUMULATE_OVERFLOW;
   }
   if (reason != MAX_NEGATIVE_SAMPLE_REASONS) {
     UMA_HISTOGRAM_ENUMERATION("UMA.NegativeSamples.Reason", reason,
                               MAX_NEGATIVE_SAMPLE_REASONS);
+    UMA_HISTOGRAM_CUSTOM_COUNTS("UMA.NegativeSamples.Increment", count, 1,
+                                1 << 30, 100);
+    UMA_HISTOGRAM_SPARSE_SLOWLY("UMA.NegativeSamples.Histogram",
+                                static_cast<int32_t>(id()));
   }
 #endif
   IncreaseSumAndCount(strict_cast<int64_t>(count) * value, count);
 }
 
 Count PersistentSampleMap::GetCount(Sample value) const {
   // Have to override "const" to make sure all samples have been loaded before
   // being able to know what value to return.
   Count* count_pointer =
       const_cast<PersistentSampleMap*>(this)->GetSampleCountStorage(value);
@@ skipping 158 matching lines @@
         found_count = &record->count;
       if (!import_everything)
         break;
     }
   }
 
   return found_count;
 }
 
 }  // namespace base