Add mode to print statistics for a given perf data file

To find fitting values for some magic heuristic values we will need
in the follow-up commits to sort events by time, this mode analyzes
a data file and computes some statistics. Unwinding and similarly
expensive operations are disabled in the `--print-stats` mode. The
output of this mode is e.g.:

~~~~~~~~~~~~~~
samples: 20210
mmaps: 24330
max buffer size: 1057128
max time: 628782799569406
max reorder time: 376374129
~~~~~~~~~~~~~~

Change-Id: I5d1344618925502b08ba303239a75d9945d965e7
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

3 files changed, 74 insertions, 8 deletions

diff --git a/app/main.cpp b/app/main.cpp
index e679f0c..8dc878c 100644
--- a/app/main.cpp
+++ b/app/main.cpp
@@ -152,6 +152,11 @@ int main(int argc, char *argv[])
                                QLatin1String("/proc/kallsyms"));
     parser.addOption(kallsymsPath);
 
+    QCommandLineOption printStats(QLatin1String("print-stats"),
+                               QCoreApplication::translate(
+                                   "main", "Print statistics instead of converting the data."));
+    parser.addOption(printStats);
+
     parser.process(app);
 
     QScopedPointer<QFile> outfile;
@@ -183,7 +188,8 @@ int main(int argc, char *argv[])
                           parser.value(debug) : parser.value(sysroot) + parser.value(debug),
                       parser.value(extra), parser.value(appPath), parser.isSet(kallsymsPath)
                         ? parser.value(kallsymsPath)
-                        : parser.value(sysroot) + parser.value(kallsymsPath));
+                        : parser.value(sysroot) + parser.value(kallsymsPath),
+                      parser.isSet(printStats));
 
     PerfHeader header(infile.data());
     PerfAttributes attributes;
diff --git a/app/perfunwind.cpp b/app/perfunwind.cpp
index 8dfcc7f..006df5e 100644
--- a/app/perfunwind.cpp
+++ b/app/perfunwind.cpp
@@ -44,13 +44,22 @@ bool operator==(const PerfUnwind::Location &a, const PerfUnwind::Location &b)
             && a.column == b.column;
 }
 
+void PerfUnwind::Stats::addEventTime(quint64 time)
+{
+    if (time && time < maxTime)
+        maxReorderTime = std::max(maxReorderTime, maxTime - time);
+    else
+        maxTime = time;
+}
+
 PerfUnwind::PerfUnwind(QIODevice *output, const QString &systemRoot, const QString &debugPath,
                        const QString &extraLibsPath, const QString &appPath,
-                       const QString &kallsymsPath) :
+                       const QString &kallsymsPath, bool printStats) :
     m_output(output), m_architecture(PerfRegisterInfo::ARCH_INVALID), m_systemRoot(systemRoot),
     m_extraLibsPath(extraLibsPath), m_appPath(appPath), m_kallsyms(kallsymsPath),
     m_sampleBufferSize(0)
 {
+    m_stats.enabled = printStats;
     m_currentUnwind.unwind = this;
     m_offlineCallbacks.find_elf = dwfl_build_id_find_elf;
     m_offlineCallbacks.find_debuginfo =  dwfl_standard_find_debuginfo;
@@ -63,11 +72,13 @@ PerfUnwind::PerfUnwind(QIODevice *output, const QString &systemRoot, const QStri
     std::memcpy(m_debugInfoPath, newDebugInfo.data(), newDebugInfo.length());
     m_offlineCallbacks.debuginfo_path = &m_debugInfoPath;
 
-    // Write minimal header, consisting of magic and data stream version we're going to use.
-    const char magic[] = "QPERFSTREAM";
-    output->write(magic, sizeof(magic));
-    qint32 dataStreamVersion = qToLittleEndian(QDataStream::Qt_DefaultCompiledVersion);
-    output->write(reinterpret_cast<const char *>(&dataStreamVersion), sizeof(qint32));
+    if (!printStats) {
+        // Write minimal header, consisting of magic and data stream version we're going to use.
+        const char magic[] = "QPERFSTREAM";
+        output->write(magic, sizeof(magic));
+        qint32 dataStreamVersion = qToLittleEndian(QDataStream::Qt_DefaultCompiledVersion);
+        output->write(reinterpret_cast<const char *>(&dataStreamVersion), sizeof(qint32));
+    }
 }
 
 PerfUnwind::~PerfUnwind()
@@ -77,6 +88,15 @@ PerfUnwind::~PerfUnwind()
 
     delete[] m_debugInfoPath;
     qDeleteAll(m_symbolTables);
+
+    if (m_stats.enabled) {
+        QTextStream out(m_output);
+        out << "samples: " << m_stats.numSamples << "\n";
+        out << "mmaps: " << m_stats.numMmaps << "\n";
+        out << "max buffer size: " << m_stats.maxBufferSize << "\n";
+        out << "max time: " << m_stats.maxTime << "\n";
+        out << "max reorder time: " << m_stats.maxReorderTime << "\n";
+    }
 }
 
 PerfSymbolTable *PerfUnwind::symbolTable(quint32 pid)
@@ -94,11 +114,20 @@ Dwfl *PerfUnwind::dwfl(quint32 pid, quint64 timestamp)
 
 void PerfUnwind::registerElf(const PerfRecordMmap &mmap)
 {
+    if (m_stats.enabled) {
+        m_stats.addEventTime(mmap.time());
+        ++m_stats.numMmaps;
+        return;
+    }
+
     symbolTable(mmap.pid())->registerElf(mmap, m_appPath, m_systemRoot, m_extraLibsPath);
 }
 
 void PerfUnwind::sendBuffer(const QByteArray &buffer)
 {
+    if (m_stats.enabled)
+        return;
+
     quint32 size = qToLittleEndian(buffer.length());
     m_output->write(reinterpret_cast<char *>(&size), sizeof(quint32));
     m_output->write(buffer);
@@ -312,6 +341,13 @@ void PerfUnwind::sample(const PerfRecordSample &sample)
     m_sampleBuffer.append(sample);
     m_sampleBufferSize += sample.size();
 
+    if (m_stats.enabled) {
+        m_stats.maxBufferSize = std::max(m_sampleBufferSize, m_stats.maxBufferSize);
+        m_stats.addEventTime(sample.time());
+        ++m_stats.numSamples;
+        // don't return early, stats should include our buffer behavior
+    }
+
     while (m_sampleBufferSize > s_maxSampleBufferSize) {
         const PerfRecordSample &sample = m_sampleBuffer.front();
         m_sampleBufferSize -= sample.size();
@@ -322,6 +358,9 @@ void PerfUnwind::sample(const PerfRecordSample &sample)
 
 void PerfUnwind::analyze(const PerfRecordSample &sample)
 {
+    if (m_stats.enabled) // don't do any time intensive work in stats mode
+        return;
+
     m_currentUnwind.isInterworking = false;
     m_currentUnwind.firstGuessedFrame = -1;
     m_currentUnwind.sample = &sample;
diff --git a/app/perfunwind.h b/app/perfunwind.h
index 931a552..b1e5599 100644
--- a/app/perfunwind.h
+++ b/app/perfunwind.h
@@ -104,7 +104,7 @@ public:
 
     PerfUnwind(QIODevice *output, const QString &systemRoot, const QString &debugInfo,
                const QString &extraLibs, const QString &appPath,
-               const QString &kallsymsPath);
+               const QString &kallsymsPath, bool printStats);
     ~PerfUnwind();
 
     PerfRegisterInfo::Architecture architecture() const { return m_architecture; }
@@ -191,6 +191,27 @@ private:
 
     static const uint s_maxSampleBufferSize = 1024 * 1024;
 
+    struct Stats
+    {
+        Stats()
+            : numSamples(0), numMmaps(0),
+            maxBufferSize(0),
+            maxTime(0), maxReorderTime(0),
+            enabled(false)
+        {}
+
+        void addEventTime(quint64 time);
+        void finishedRound();
+
+        quint64 numSamples;
+        quint64 numMmaps;
+        uint maxBufferSize;
+        quint64 maxTime;
+        quint64 maxReorderTime;
+        bool enabled;
+    };
+    Stats m_stats;
+
     void unwindStack(Dwfl *dwfl);
     void resolveCallchain();
     void analyze(const PerfRecordSample &sample);