[PATCH V5 14/14] perf, tools: Add documentation for topdown metrics

kan.liang
2019-11-03T23:30:50Z

From: Andi Kleen <ak@linux.intel.com>

Add some documentation how to use the topdown metrics in ring 3.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---

Changes since V4
- Update example for Ice Lake

tools/perf/Documentation/topdown.txt | 235 +++++++++++++++++++++++++++
1 file changed, 235 insertions(+)
create mode 100644 tools/perf/Documentation/topdown.txt

diff --git a/tools/perf/Documentation/topdown.txt b/tools/perf/Documentation/topdown.txt
new file mode 100644
index 000000000000..e724d2af3b8d
--- /dev/null
+++ b/tools/perf/Documentation/topdown.txt
@@ -0,0 +1,235 @@
+Using TopDown metrics in user space
+-----------------------------------
+
+Intel CPUs (since Sandy Bridge and Silvermont) support a TopDown
+methology to break down CPU pipeline execution into 4 bottlenecks:
+frontend bound, backend bound, bad speculation, retiring.
+
+For more details on Topdown see [1][5]
+
+Traditionally this was implemented by events in generic counters
+and specific formulas to compute the bottlenecks.
+
+perf stat --topdown implements this.
+
+Full Top Down includes more levels that can break down the
+bottlenecks further. This is not directly implemented in perf,
+but available in other tools that can run on top of perf,
+such as toplev[2] or vtune[3]
+
+New Topdown features in Ice Lake
+===============================
+
+With Ice Lake CPUs the TopDown metrics are directly available as
+fixed counters and do not require generic counters. This allows
+to collect TopDown always in addition to other events.
+
+% perf stat -a --topdown -I1000
+# time counts unit events
+ 1.000854735 20,097,158,100 slots
+ 1.000854735 79,327,616 topdown-retiring # 0.4% retiring
+ 1.000854735 157,932,715 topdown-bad-spec # 0.8% bad speculation
+ 1.000854735 81,610,855 topdown-fe-bound # 0.4% frontend bound
+ 1.000854735 19,778,286,903 topdown-be-bound # 98.4% backend bound
+ 2.003623823 20,010,908,365 slots
+ 2.003623823 79,905,340 topdown-retiring # 0.4% retiring
+ 2.003623823 158,405,024 topdown-bad-spec # 0.8% bad speculation
+ 2.003623823 87,980,097 topdown-fe-bound # 0.4% frontend bound
+ 2.003623823 19,684,617,888 topdown-be-bound # 98.4% backend bound
+ 3.005828889 20,062,101,220 slots
+ 3.005828889 80,077,032 topdown-retiring # 0.4% retiring
+ 3.005828889 158,682,921 topdown-bad-spec # 0.8% bad speculation
+ 3.005828889 86,579,604 topdown-fe-bound # 0.4% frontend bound
+ 3.005828889 19,736,761,649 topdown-be-bound # 98.4% backend bound
+...
+
+This also enables measuring TopDown per thread/process instead
+of only per core.
+
+Using TopDown through RDPMC in applications on Ice Lake
+======================================================
+
+For more fine grained measurements it can be useful to
+access the new directly from user space. This is more complicated,
+but drastically lowers overhead.
+
+On Ice Lake, there is a new fixed counter 3: SLOTS, which reports
+"pipeline SLOTS" (cycles multiplied by core issue width) and a
+metric register that reports slots ratios for the different bottleneck
+categories.
+
+The metrics counter is CPU model specific and is not be available
+on older CPUs.
+
+Example code
+============
+
+Library functions to do the functionality described below
+is also available in libjevents [4]
+
+The application opens a perf_event file descriptor
+and sets up fixed counter 3 (SLOTS) to start and
+allow user programs to read the performance counters.
+
+Fixed counter 3 is mapped to a pseudo event event=0x00, umask=04,
+so the perf_event_attr structure should be initialized with
+{ .config = 0x0400, .type = PERF_TYPE_RAW }
+
+#include <linux/perf_event.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+/* Provide own perf_event_open stub because glibc doesn't */
+__attribute__((weak))
+int perf_event_open(struct perf_event_attr *attr, pid_t pid,
+ int cpu, int group_fd, unsigned long flags)
+{
+ return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags);
+}
+
+/* open slots counter file descriptor for current task */
+struct perf_event_attr slots = {
+ .type = PERF_TYPE_RAW,
+ .size = sizeof(struct perf_event_attr),
+ .config = 0x400,
+ .exclude_kernel = 1,
+};
+
+int fd = perf_event_open(&slots, 0, -1, -1, 0);
+if (fd < 0)
+ ... error ...
+
+The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used
+to read slots and the topdown metrics at different points of the program:
+
+#include <stdint.h>
+#include <x86intrin.h>
+
+#define RDPMC_FIXED (1 << 30) /* return fixed counters */
+#define RDPMC_METRIC (1 << 29) /* return metric counters */
+
+#define FIXED_COUNTER_SLOTS 3
+#define METRIC_COUNTER_TOPDOWN_L1 0
+
+static inline uint64_t read_slots(void)
+{
+ return _rdpmc(RDPMC_FIXED | FIXED_COUNTER_SLOTS);
+}
+
+static inline uint64_t read_metrics(void)
+{
+ return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1);
+}
+
+Then the program can be instrumented to read these metrics at different
+points.
+
+It's not a good idea to do this with too short code regions,
+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.
+
+Decoding metrics values
+=======================
+
+The value reported by read_metrics() contains four 8 bit fields
+that represent a scaled ratio that represent the Level 1 bottleneck.
+All four fields add up to 0xff (= 100%)
+
+The binary ratios in the metric value can be converted to float ratios:
+
+#define GET_METRIC(m, i) (((m) >> (i*8)) & 0xff)
+
+#define TOPDOWN_RETIRING(val) ((float)GET_METRIC(val, 0) / 0xff)
+#define TOPDOWN_BAD_SPEC(val) ((float)GET_METRIC(val, 1) / 0xff)
+#define TOPDOWN_FE_BOUND(val) ((float)GET_METRIC(val, 2) / 0xff)
+#define TOPDOWN_BE_BOUND(val) ((float)GET_METRIC(val, 3) / 0xff)
+
+and then converted to percent for printing.
+
+The ratios in the metric accumulate for the time when the counter
+is enabled. For measuring programs it is often useful to measure
+specific sections. For this it is needed to deltas on metrics.
+
+This can be done by scaling the metrics with the slots counter
+read at the same time.
+
+Then it's possible to take deltas of these slots counts
+measured at different points, and determine the metrics
+for that time period.
+
+ slots_a = read_slots();
+ metric_a = read_metrics();
+
+ ... larger code region ...
+
+ slots_b = read_slots()
+ metric_b = read_metrics()
+
+ # compute scaled metrics for measurement a
+ retiring_slots_a = GET_METRIC(metric_a, 0) * slots_a
+ bad_spec_slots_a = GET_METRIC(metric_a, 1) * slots_a
+ fe_bound_slots_a = GET_METRIC(metric_a, 2) * slots_a
+ be_bound_slots_a = GET_METRIC(metric_a, 3) * slots_a
+
+ # compute delta scaled metrics between b and a
+ retiring_slots = GET_METRIC(metric_b, 0) * slots_b - retiring_slots_a
+ bad_spec_slots = GET_METRIC(metric_b, 1) * slots_b - bad_spec_slots_a
+ fe_bound_slots = GET_METRIC(metric_b, 2) * slots_b - fe_bound_slots_a
+ be_bound_slots = GET_METRIC(metric_b, 3) * slots_b - be_bound_slots_a
+
+Later the individual ratios for the measurement period can be recreated
+from these counts.
+
+ slots_delta = slots_b - slots_a
+ retiring_ratio = (float)retiring_slots / slots_delta
+ bad_spec_ratio = (float)bad_spec_slots / slots_delta
+ fe_bound_ratio = (float)fe_bound_slots / slots_delta
+ be_bound_ratio = (float)be_bound_slots / slota_delta
+
+ printf("Retiring %.2f%% Bad Speculation %.2f%% FE Bound %.2f%% BE Bound %.2f%%\n",
+ retiring_ratio * 100.,
+ bad_spec_ratio * 100.,
+ fe_bound_ratio * 100.,
+ be_bound_ratio * 100.);
+
+Resetting metrics counters
+==========================
+
+Since the individual metrics are only 8bit they lose precision for
+short regions over time because the number of cycles covered by each
+fraction bit shrinks. So the counters need to be reset regularly.
+
+When using the kernel perf API the kernel resets on every read.
+So as long as the reading is at reasonable intervals (every few
+seconds) the precision is good.
+
+When using perf stat it is recommended to always use the -I option,
+with no longer interval than a few seconds
+
+ perf stat -I 1000 --topdown ...
+
+For user programs using RDPMC directly the counter can
+be reset explicitly using ioctl:
+
+ ioctl(perf_fd, PERF_EVENT_IOC_RESET, 0);
+
+This "opens" a new measurement period.
+
+A program using RDPMC for TopDown should schedule such a reset
+regularly, as in every few seconds.
+
+Limits on Ice Lake
+==================
+
+All the TopDown events must be in a group with SLOTS events.
+
+There is no sampling support for TopDown events.
+Sampling read SLOTS and TopDown events is forbidden.
+For example, perf record -e '{slots, topdown-retiring}:S'
+
+[1] https://software.intel.com/en-us/top-down-microarchitecture-analysis-method-win
+[2] https://github.com/andikleen/pmu-tools/wiki/toplev-manual
+[3] https://software.intel.com/en-us/intel-vtune-amplifier-xe
+[4] https://github.com/andikleen/pmu-tools/tree/master/jevents
+[5] https://sites.google.com/site/analysismethods/yasin-pubs
--
2.17.1