The Fixed counter 3 must be the leader of the group.
#include <linux/perf_event.h>
+#include <sys/mman.h>
#include <sys/syscall.h>
#include <unistd.h>
if (slots_fd < 0)
... error ...
+/* Memory mapping the fd permits _rdpmc calls from userspace */
+void *slots_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, slots_fd, 0);
+if (!slot_p)
+ .... error ...
+
/*
* Open metrics event file descriptor for current task.
* Set slots event as the leader of the group.
if (metrics_fd < 0)
... error ...
+/* Memory mapping the fd permits _rdpmc calls from userspace */
+void *metrics_p = mmap(0, getpagesize(), PROT_READ, MAP_SHARED, metrics_fd, 0);
+if (!metrics_p)
+ ... error ...
+
+Note: the file descriptors returned by the perf_event_open calls must be memory
+mapped to permit calls to the _rdpmd instruction. Permission may also be granted
+by writing the /sys/devices/cpu/rdpmc sysfs node.
The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used
to read slots and the topdown metrics at different points of the program:
cause too much measurement inaccuracy. For example instrumenting
individual basic blocks is definitely too fine grained.
+_rdpmc calls should not be mixed with reading the metrics and slots counters
+through system calls, as the kernel will reset these counters after each system
+call.
+
Decoding metrics values
=======================