diff options
Diffstat (limited to 'tools/perf/Documentation/topdown.txt')
| -rw-r--r-- | tools/perf/Documentation/topdown.txt | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/tools/perf/Documentation/topdown.txt b/tools/perf/Documentation/topdown.txt index 10f07f9455b8..c6302df4cf29 100644 --- a/tools/perf/Documentation/topdown.txt +++ b/tools/perf/Documentation/topdown.txt @@ -72,6 +72,7 @@ For example, the perf_event_attr structure can be initialized with 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> @@ -95,6 +96,11 @@ int slots_fd = perf_event_open(&slots, 0, -1, -1, 0); 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. @@ -110,6 +116,14 @@ int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0); 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: @@ -141,6 +155,10 @@ as the parallelism and overlap in the CPU program execution will 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 ======================= |