Android: Code residency: Measure success rate of the measurements

Introduce a histogram reporting successes and failures at pulling code
residency data from /proc/self/pagemap.

Opening and reading /proc/self/pagemap can fail at fopen(3), fseek(3)
and fread(3). We think that we can potentially mitigate errors at fopen,
see the bug for details.

The current code distinguishes two error kinds:
1. at fopen
2. at fseek or fread

The histogram mirrors this separation. In the future the fopen errors
can also be broken down into several errno values. At this point the
extra detail seems unnecessary.

Bug: 1070618
Change-Id: I2b88c24c0861b46845707dbf88b16b2e4817ae22
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2215856
Commit-Queue: Egor Pasko <pasko@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772314}

services/resource_coordinator/public/cpp/memory_instrumentation/os_metrics.h

@@ -45,21 +45,32 @@ class COMPONENT_EXPORT(
 #endif
 
 #if defined(OS_LINUX) || defined(OS_ANDROID)
-  // Provides information on the dump state of resident pages.
+  // Provides information on the dump state of resident pages. These values are
+  // written to logs. New enum values can be added, but existing enums must
+  // never be renumbered or deleted and reused.
   enum class MappedAndResidentPagesDumpState {
     // Access to /proc/<pid>/pagemap can be denied for android devices running
     // a kernel version < 4.4.
-    kAccessPagemapDenied,
-    kFailure,
-    kSuccess
+    kAccessPagemapDenied = 0,
+    kFailure = 1,
+    kSuccess = 2,
+
+    // Must be equal to the greatest among enumeraiton values.
+    kMaxValue = kSuccess
   };
 
-  // Depends on /proc/self/pagemap to determine mapped and resident pages
-  // within bounds (start_address inclusive and end_address exclusive).
-  // It does not use mincore() because it only checks to see
-  // if the page is in the cache and up to date.
-  // mincore() has no guarantee a page has been mapped by the current process.
-  // Guaranteed to work on Android.
+  // Fills out a bitmap of memory pages accessed by the current process that are
+  // still in pagecache.
+  //
+  // Depends on /proc/self/pagemap to determine the mapped and resident pages
+  // within bounds (|start_address| inclusive and |end_address| exclusive).
+  //
+  // Does not use mincore() because the latter only reports resident pages. The
+  // mincore() would report a page as resident if that page was accessed from a
+  // different process (such as the commonly used prefetch of the native
+  // library).
+  //
+  // Tested only on Android.
   static MappedAndResidentPagesDumpState GetMappedAndResidentPages(
       const size_t start_address,
       const size_t end_address,

services/resource_coordinator/public/cpp/memory_instrumentation/os_metrics_linux.cc

@@ -14,6 +14,7 @@
 #include "base/files/file_util.h"
 #include "base/files/scoped_file.h"
 #include "base/format_macros.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/process/process_metrics.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
@@ -280,9 +281,13 @@ bool OSMetrics::FillOSMemoryDump(base::ProcessId pid,
       OSMetrics::GetMappedAndResidentPages(base::android::kStartOfText,
                                            base::android::kEndOfText,
                                            &accessed_pages_bitmap);
+  UMA_HISTOGRAM_ENUMERATION(
+      "Memory.NativeLibrary.MappedAndResidentMemoryFootprintCollectionStatus",
+      state);
 
   // MappedAndResidentPagesDumpState |state| can be |kAccessPagemapDenied|
-  // for Android devices running a kernel version < 4.4.
+  // for Android devices running a kernel version < 4.4 or because the process
+  // is not "dumpable", as described in proc(5).
   if (state != OSMetrics::MappedAndResidentPagesDumpState::kSuccess)
     return state != OSMetrics::MappedAndResidentPagesDumpState::kFailure;
 

tools/metrics/histograms/enums.xml

@@ -42955,6 +42955,12 @@ from previous Chrome versions.
   <int value="12" label="Bad Update specification"/>
 </enum>
 
+<enum name="MappedAndResidentPagesDumpState">
+  <int value="0" label="kAccessPagemapDenied"/>
+  <int value="1" label="kFailure"/>
+  <int value="2" label="kSuccess"/>
+</enum>
+
 <enum name="MappedCSSProperties">
 <!-- Generated from third_party/blink/public/mojom/use_counter/css_property_id.mojom.
 Called by update_use_counter_css.py.-->

tools/metrics/histograms/histograms.xml

@@ -82163,6 +82163,20 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
+<histogram
+    name="Memory.NativeLibrary.MappedAndResidentMemoryFootprintCollectionStatus"
+    enum="MappedAndResidentPagesDumpState" expires_after="2020-10-11">
+  <owner>lizeb@chromium.org</owner>
+  <owner>pasko@chromium.org</owner>
+  <summary>
+    Result of an attempt to read /proc/self/pagemap when determining the amount
+    of resident memory mapped by the current process. Reading the file is
+    performed as part of computing the
+    Memory.NativeLibrary.MappedAndResidentMemoryFootprint2 histogram. Recorded
+    once per UMA ping. Available only on Android.
+  </summary>
+</histogram>
+
 <histogram name="Memory.NativeLibrary.NotResidentOrderedCodeMemoryFootprint"
     units="KB" expires_after="2020-10-11">
   <owner>lizeb@chromium.org</owner>