TabManager: Take min_free_kbytes into consideration when killing tabs.

On ChromeOS, kernel notifies Chrome when system gets into low memory
condition. However the low memory calculation misses a parameter
|min_free_kbytes| so the notification comes too late. A kernel fix is
coming to fix the issue.

On Chrome side, TabManager tries to recover memory to normal level. It
mirrors the low memory calculation in kernel to know how many tabs to
kill. So the missing parameter |min_free_kbytes| should be added back.

In the future, try to consolidate the logic in kernel so TabManager
doesn't need to do its own calculation.

BUG=chromium:645512,chrome-os-partner:60498
TEST=cave,peppy,etc.

Review-Url: https://codereview.chromium.org/2618483002
Cr-Commit-Position: refs/heads/master@{#442018}

chrome/browser/memory/tab_manager_delegate_chromeos.cc

@@ -249,16 +249,24 @@ int TabManagerDelegate::MemoryStat::LowMemoryMarginKB() {
 int TabManagerDelegate::MemoryStat::TargetMemoryToFreeKB() {
   static const int kRamVsSwapWeight = 4;
   static const char kMinFilelistConfig[] = "/proc/sys/vm/min_filelist_kbytes";
+  static const char kMinFreeKbytes[] = "/proc/sys/vm/min_free_kbytes";
 
   base::SystemMemoryInfoKB system_mem;
   base::GetSystemMemoryInfo(&system_mem);
   const int file_mem_kb = system_mem.active_file + system_mem.inactive_file;
   const int min_filelist_kb = ReadIntFromFile(kMinFilelistConfig, 0);
+  const int min_free_kb = ReadIntFromFile(kMinFreeKbytes, 0);
   // Calculate current available memory in system.
   // File-backed memory should be easy to reclaim, unless they're dirty.
+  // TODO(cylee): On ChromeOS, kernel reports low memory condition when
+  // available memory is low. The following formula duplicates the logic in
+  // kernel to calculate how much memory should be released. In the future,
+  // kernel should try to report the amount of memory to release directly to
+  // eliminate the duplication here.
   const int available_mem_kb = system_mem.free +
       file_mem_kb - system_mem.dirty - min_filelist_kb +
-      system_mem.swap_free / kRamVsSwapWeight;
+      system_mem.swap_free / kRamVsSwapWeight -
+      min_free_kb;
 
   return LowMemoryMarginKB() - available_mem_kb;
 }
@@ -573,7 +581,8 @@ void TabManagerDelegate::LowMemoryKillImpl(
     const TabStatsList& tab_list,
     const std::vector<arc::ArcProcess>& arc_processes) {
 
-  VLOG(2) << "LowMemoryKilleImpl";
+  VLOG(2) << "LowMemoryKillImpl";
+
   const std::vector<TabManagerDelegate::Candidate> candidates =
       GetSortedCandidates(tab_list, arc_processes);