Implement the Linux Perf Counter backend for benchlib

Currently, we only support one event type: counting CPU cycles with
hardware counters. There are no fallbacks if this hardware counter is
not available, and there is currently no way to specify other
counters.

Benchlib only supports reporting one event per benchmark, even though
the event counter interface allows specifying more than one. Still,
the hardware usually has limitations on how many events it can monitor
at a time, and we'd prefer to have the counter running at 100% of the
time, so this will not change.

Change-Id: I79858a3ad1e696dc4b7b72c420e5a04b67cd55de
Reviewed-by: Jason McDonald <macadder1@gmail.com>

src/testlib/qbenchmarkperfevents.cpp

@@ -44,15 +44,41 @@
 
 #ifdef QTESTLIB_USE_PERF_EVENTS
 
+// include the qcore_unix_p.h without core-private
+// we only use inline functions anyway
+#include "../corelib/kernel/qcore_unix_p.h"
+
 #include <sys/types.h>
 #include <errno.h>
+#include <fcntl.h>
+#include <string.h>
 
 #include <sys/syscall.h>
+#include <sys/ioctl.h>
 
 #include "3rdparty/linux_perf_event_p.h"
 
 QT_BEGIN_NAMESPACE
 
+/*!
+    \class QBenchmarkPerfEvents
+    \brief The Linux perf events benchmark backend
+
+    This benchmark backend uses the Linux Performance Counters interface,
+    introduced with the Linux kernel v2.6.31. The interface is done by one
+    system call (perf_event_open) which takes an attribute structure and
+    returns a file descriptor.
+
+    More information:
+     \li design docs: tools/perf/design.txt <http://lxr.linux.no/linux/tools/perf/design.txt>
+     \li sample tool: tools/perf/builtin-stat.c <http://lxr.linux.no/linux/tools/perf/builtin-stat.c>
+    (note: as of v3.3.1, the documentation is out-of-date with the kernel
+    interface, so reading the source code of existing tools is necessary)
+
+    This benchlib backend monitors the current process as well as child process
+    launched. We do not try to benchmark in kernel or hypervisor mode, as that
+    usually requires elevated privileges.
+ */
 
 static int perf_event_open(perf_event_attr *attr, pid_t pid, int cpu, int group_fd, unsigned long flags)
 {
@@ -67,27 +93,69 @@ bool QBenchmarkPerfEventsMeasurer::isAvailable()
 }
 
 QBenchmarkPerfEventsMeasurer::QBenchmarkPerfEventsMeasurer()
+    : fd(-1)
 {
 }
 
 QBenchmarkPerfEventsMeasurer::~QBenchmarkPerfEventsMeasurer()
 {
+    qt_safe_close(fd);
 }
 
 void QBenchmarkPerfEventsMeasurer::init()
 {
+    perf_event_attr attr;
+    memset(&attr, 0, sizeof attr);
+
+    // common init
+    attr.size = sizeof attr;
+    attr.sample_period = 0;
+    attr.sample_type = 0;
+    attr.read_format = PERF_FORMAT_TOTAL_TIME_ENABLED | PERF_FORMAT_TOTAL_TIME_RUNNING;
+    attr.disabled = true; // start disabled, we'll enable later
+    attr.inherit = true; // let children inherit, if the benchmark has child processes
+    attr.pinned = true; // keep it running on the PMU
+    attr.inherit_stat = true; // collapse all the info from child processes
+    attr.task = true; // trace fork and exit
+
+    // our event type
+    // ### FIXME hardcoded for now
+    attr.type = PERF_TYPE_HARDWARE;
+    attr.config = PERF_COUNT_HW_CPU_CYCLES;
+
+    // pid == 0 -> attach to the current process
+    // cpu == -1 -> monitor on all CPUs
+    // group_fd == -1 -> this is the group leader
+    // flags == 0 -> reserved, must be zero
+    fd = perf_event_open(&attr, 0, -1, -1, 0);
+    if (fd == -1) {
+        perror("QBenchmarkPerfEventsMeasurer::start: perf_event_open");
+        exit(1);
+    } else {
+        ::fcntl(fd, F_SETFD, FD_CLOEXEC);
+    }
 }
 
 void QBenchmarkPerfEventsMeasurer::start()
 {
+    // enable the counter
+    ::ioctl(fd, PERF_EVENT_IOC_RESET);
+    ::ioctl(fd, PERF_EVENT_IOC_ENABLE);
 }
 
 qint64 QBenchmarkPerfEventsMeasurer::checkpoint()
 {
+    ::ioctl(fd, PERF_EVENT_IOC_DISABLE);
+    qint64 value = readValue();
+    ::ioctl(fd, PERF_EVENT_IOC_ENABLE);
+    return value;
 }
 
 qint64 QBenchmarkPerfEventsMeasurer::stop()
 {
+    // disable the counter
+    ::ioctl(fd, PERF_EVENT_IOC_DISABLE);
+    return readValue();
 }
 
 bool QBenchmarkPerfEventsMeasurer::isMeasurementAccepted(qint64)
@@ -110,6 +178,41 @@ QTest::QBenchmarkMetric QBenchmarkPerfEventsMeasurer::metricType()
     return QTest::Events;
 }
 
-#endif
+qint64 QBenchmarkPerfEventsMeasurer::readValue()
+{
+    /* from the kernel docs:
+     * struct read_format {
+     *  { u64           value;
+     *    { u64         time_enabled; } && PERF_FORMAT_TOTAL_TIME_ENABLED
+     *    { u64         time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
+     *    { u64         id;           } && PERF_FORMAT_ID
+     *  } && !PERF_FORMAT_GROUP
+     */
+
+    struct read_format {
+        quint64 value;
+        quint64 time_enabled;
+        quint64 time_running;
+    } results;
+
+    size_t nread = 0;
+    while (nread < sizeof results) {
+        char *ptr = reinterpret_cast<char *>(&results);
+        qint64 r = qt_safe_read(fd, ptr + nread, sizeof results - nread);
+        if (r == -1) {
+            perror("QBenchmarkPerfEventsMeasurer::readValue: reading the results");
+            exit(1);
+        }
+        nread += quint64(r);
+    }
+
+    if (results.time_running == results.time_enabled)
+        return results.value;
+
+    // scale the results, though this shouldn't happen!
+    return results.value * (double(results.time_running) / double(results.time_enabled));
+}
 
 QT_END_NAMESPACE
+
+#endif

src/testlib/qbenchmarkperfevents_p.h

@@ -75,6 +75,9 @@ public:
 
     static bool isAvailable();
 private:
+    int fd;
+
+    qint64 readValue();
 };
 
 QT_END_NAMESPACE

src/testlib/qtestcase.cpp

@@ -1487,7 +1487,8 @@ Q_TESTLIB_EXPORT void qtest_qParseArgs(int argc, char *argv[], bool qml)
 #ifdef QTESTLIB_USE_PERF_EVENTS
         } else if (strcmp(argv[i], "-perf") == 0) {
             if (QBenchmarkPerfEventsMeasurer::isAvailable()) {
-                printf("perf available\n");
+                // perf available
+                QBenchmarkGlobalData::current->setMode(QBenchmarkGlobalData::PerfCounter);
             } else {
                 fprintf(stderr, "WARNING: Linux perf events not available. Using the walltime measurer.\n");
             }