Kernel+Profiler: Track lost time between profiler timer ticks

We can lose profiling timer events for a few reasons, for example
disabled interrupts or system slowness. This accounts for lost
time between CPU samples by adding a field lost_samples to each
profiling event which tracks how many samples were lost immediately
preceding the event.

Userland/DevTools/Profiler/Profile.cpp

@@ -228,6 +228,7 @@ Result<NonnullOwnPtr<Profile>, String> Profile::load_from_perfcore_file(const St
         Event event;
 
         event.timestamp = perf_event.get("timestamp").to_number<u64>();
+        event.lost_samples = perf_event.get("lost_samples").to_number<u32>();
         event.type = perf_event.get("type").to_string();
         event.pid = perf_event.get("pid").to_i32();
         event.tid = perf_event.get("tid").to_i32();

Userland/DevTools/Profiler/Profile.h

@@ -166,6 +166,7 @@ public:
         String executable;
         int pid { 0 };
         int tid { 0 };
+        u32 lost_samples { 0 };
         bool in_kernel { false };
         Vector<Frame> frames;
     };

Userland/DevTools/Profiler/SamplesModel.cpp

@@ -45,6 +45,8 @@ String SamplesModel::column_name(int column) const
         return "TID";
     case Column::ExecutableName:
         return "Executable";
+    case Column::LostSamples:
+        return "Lost Samples";
     case Column::InnermostStackFrame:
         return "Innermost Frame";
     default:
@@ -81,6 +83,10 @@ GUI::Variant SamplesModel::data(const GUI::ModelIndex& index, GUI::ModelRole rol
             return (u32)event.timestamp;
         }
 
+        if (index.column() == Column::LostSamples) {
+            return event.lost_samples;
+        }
+
         if (index.column() == Column::InnermostStackFrame) {
             return event.frames.last().symbol;
         }

Userland/DevTools/Profiler/SamplesModel.h

@@ -25,6 +25,7 @@ public:
         ProcessID,
         ThreadID,
         ExecutableName,
+        LostSamples,
         InnermostStackFrame,
         __Count
     };

Userland/DevTools/Profiler/TimelineTrack.cpp

@@ -78,7 +78,7 @@ void TimelineTrack::paint_event(GUI::PaintEvent& event)
         }
 
         auto& histogram = event.in_kernel ? kernel_histogram : usermode_histogram;
-        histogram.insert(clamp_timestamp(event.timestamp), 1);
+        histogram.insert(clamp_timestamp(event.timestamp), 1 + event.lost_samples);
     }
 
     decltype(kernel_histogram.at(0)) max_value = 0;