1Using TopDown metrics in user space 2----------------------------------- 3 4Intel CPUs (since Sandy Bridge and Silvermont) support a TopDown 5methology to break down CPU pipeline execution into 4 bottlenecks: 6frontend bound, backend bound, bad speculation, retiring. 7 8For more details on Topdown see [1][5] 9 10Traditionally this was implemented by events in generic counters 11and specific formulas to compute the bottlenecks. 12 13perf stat --topdown implements this. 14 15Full Top Down includes more levels that can break down the 16bottlenecks further. This is not directly implemented in perf, 17but available in other tools that can run on top of perf, 18such as toplev[2] or vtune[3] 19 20New Topdown features in Ice Lake 21=============================== 22 23With Ice Lake CPUs the TopDown metrics are directly available as 24fixed counters and do not require generic counters. This allows 25to collect TopDown always in addition to other events. 26 27% perf stat -a --topdown -I1000 28# time retiring bad speculation frontend bound backend bound 29 1.001281330 23.0% 15.3% 29.6% 32.1% 30 2.003009005 5.0% 6.8% 46.6% 41.6% 31 3.004646182 6.7% 6.7% 46.0% 40.6% 32 4.006326375 5.0% 6.4% 47.6% 41.0% 33 5.007991804 5.1% 6.3% 46.3% 42.3% 34 6.009626773 6.2% 7.1% 47.3% 39.3% 35 7.011296356 4.7% 6.7% 46.2% 42.4% 36 8.012951831 4.7% 6.7% 47.5% 41.1% 37... 38 39This also enables measuring TopDown per thread/process instead 40of only per core. 41 42Using TopDown through RDPMC in applications on Ice Lake 43====================================================== 44 45For more fine grained measurements it can be useful to 46access the new directly from user space. This is more complicated, 47but drastically lowers overhead. 48 49On Ice Lake, there is a new fixed counter 3: SLOTS, which reports 50"pipeline SLOTS" (cycles multiplied by core issue width) and a 51metric register that reports slots ratios for the different bottleneck 52categories. 53 54The metrics counter is CPU model specific and is not available on older 55CPUs. 56 57Example code 58============ 59 60Library functions to do the functionality described below 61is also available in libjevents [4] 62 63The application opens a group with fixed counter 3 (SLOTS) and any 64metric event, and allow user programs to read the performance counters. 65 66Fixed counter 3 is mapped to a pseudo event event=0x00, umask=04, 67so the perf_event_attr structure should be initialized with 68{ .config = 0x0400, .type = PERF_TYPE_RAW } 69The metric events are mapped to the pseudo event event=0x00, umask=0x8X. 70For example, the perf_event_attr structure can be initialized with 71{ .config = 0x8000, .type = PERF_TYPE_RAW } for Retiring metric event 72The Fixed counter 3 must be the leader of the group. 73 74#include <linux/perf_event.h> 75#include <sys/syscall.h> 76#include <unistd.h> 77 78/* Provide own perf_event_open stub because glibc doesn't */ 79__attribute__((weak)) 80int perf_event_open(struct perf_event_attr *attr, pid_t pid, 81 int cpu, int group_fd, unsigned long flags) 82{ 83 return syscall(__NR_perf_event_open, attr, pid, cpu, group_fd, flags); 84} 85 86/* Open slots counter file descriptor for current task. */ 87struct perf_event_attr slots = { 88 .type = PERF_TYPE_RAW, 89 .size = sizeof(struct perf_event_attr), 90 .config = 0x400, 91 .exclude_kernel = 1, 92}; 93 94int slots_fd = perf_event_open(&slots, 0, -1, -1, 0); 95if (slots_fd < 0) 96 ... error ... 97 98/* 99 * Open metrics event file descriptor for current task. 100 * Set slots event as the leader of the group. 101 */ 102struct perf_event_attr metrics = { 103 .type = PERF_TYPE_RAW, 104 .size = sizeof(struct perf_event_attr), 105 .config = 0x8000, 106 .exclude_kernel = 1, 107}; 108 109int metrics_fd = perf_event_open(&metrics, 0, -1, slots_fd, 0); 110if (metrics_fd < 0) 111 ... error ... 112 113 114The RDPMC instruction (or _rdpmc compiler intrinsic) can now be used 115to read slots and the topdown metrics at different points of the program: 116 117#include <stdint.h> 118#include <x86intrin.h> 119 120#define RDPMC_FIXED (1 << 30) /* return fixed counters */ 121#define RDPMC_METRIC (1 << 29) /* return metric counters */ 122 123#define FIXED_COUNTER_SLOTS 3 124#define METRIC_COUNTER_TOPDOWN_L1 0 125 126static inline uint64_t read_slots(void) 127{ 128 return _rdpmc(RDPMC_FIXED | FIXED_COUNTER_SLOTS); 129} 130 131static inline uint64_t read_metrics(void) 132{ 133 return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1); 134} 135 136Then the program can be instrumented to read these metrics at different 137points. 138 139It's not a good idea to do this with too short code regions, 140as the parallelism and overlap in the CPU program execution will 141cause too much measurement inaccuracy. For example instrumenting 142individual basic blocks is definitely too fine grained. 143 144Decoding metrics values 145======================= 146 147The value reported by read_metrics() contains four 8 bit fields 148that represent a scaled ratio that represent the Level 1 bottleneck. 149All four fields add up to 0xff (= 100%) 150 151The binary ratios in the metric value can be converted to float ratios: 152 153#define GET_METRIC(m, i) (((m) >> (i*8)) & 0xff) 154 155#define TOPDOWN_RETIRING(val) ((float)GET_METRIC(val, 0) / 0xff) 156#define TOPDOWN_BAD_SPEC(val) ((float)GET_METRIC(val, 1) / 0xff) 157#define TOPDOWN_FE_BOUND(val) ((float)GET_METRIC(val, 2) / 0xff) 158#define TOPDOWN_BE_BOUND(val) ((float)GET_METRIC(val, 3) / 0xff) 159 160and then converted to percent for printing. 161 162The ratios in the metric accumulate for the time when the counter 163is enabled. For measuring programs it is often useful to measure 164specific sections. For this it is needed to deltas on metrics. 165 166This can be done by scaling the metrics with the slots counter 167read at the same time. 168 169Then it's possible to take deltas of these slots counts 170measured at different points, and determine the metrics 171for that time period. 172 173 slots_a = read_slots(); 174 metric_a = read_metrics(); 175 176 ... larger code region ... 177 178 slots_b = read_slots() 179 metric_b = read_metrics() 180 181 # compute scaled metrics for measurement a 182 retiring_slots_a = GET_METRIC(metric_a, 0) * slots_a 183 bad_spec_slots_a = GET_METRIC(metric_a, 1) * slots_a 184 fe_bound_slots_a = GET_METRIC(metric_a, 2) * slots_a 185 be_bound_slots_a = GET_METRIC(metric_a, 3) * slots_a 186 187 # compute delta scaled metrics between b and a 188 retiring_slots = GET_METRIC(metric_b, 0) * slots_b - retiring_slots_a 189 bad_spec_slots = GET_METRIC(metric_b, 1) * slots_b - bad_spec_slots_a 190 fe_bound_slots = GET_METRIC(metric_b, 2) * slots_b - fe_bound_slots_a 191 be_bound_slots = GET_METRIC(metric_b, 3) * slots_b - be_bound_slots_a 192 193Later the individual ratios for the measurement period can be recreated 194from these counts. 195 196 slots_delta = slots_b - slots_a 197 retiring_ratio = (float)retiring_slots / slots_delta 198 bad_spec_ratio = (float)bad_spec_slots / slots_delta 199 fe_bound_ratio = (float)fe_bound_slots / slots_delta 200 be_bound_ratio = (float)be_bound_slots / slota_delta 201 202 printf("Retiring %.2f%% Bad Speculation %.2f%% FE Bound %.2f%% BE Bound %.2f%%\n", 203 retiring_ratio * 100., 204 bad_spec_ratio * 100., 205 fe_bound_ratio * 100., 206 be_bound_ratio * 100.); 207 208Resetting metrics counters 209========================== 210 211Since the individual metrics are only 8bit they lose precision for 212short regions over time because the number of cycles covered by each 213fraction bit shrinks. So the counters need to be reset regularly. 214 215When using the kernel perf API the kernel resets on every read. 216So as long as the reading is at reasonable intervals (every few 217seconds) the precision is good. 218 219When using perf stat it is recommended to always use the -I option, 220with no longer interval than a few seconds 221 222 perf stat -I 1000 --topdown ... 223 224For user programs using RDPMC directly the counter can 225be reset explicitly using ioctl: 226 227 ioctl(perf_fd, PERF_EVENT_IOC_RESET, 0); 228 229This "opens" a new measurement period. 230 231A program using RDPMC for TopDown should schedule such a reset 232regularly, as in every few seconds. 233 234Limits on Ice Lake 235================== 236 237Four pseudo TopDown metric events are exposed for the end-users, 238topdown-retiring, topdown-bad-spec, topdown-fe-bound and topdown-be-bound. 239They can be used to collect the TopDown value under the following 240rules: 241- All the TopDown metric events must be in a group with the SLOTS event. 242- The SLOTS event must be the leader of the group. 243- The PERF_FORMAT_GROUP flag must be applied for each TopDown metric 244 events 245 246The SLOTS event and the TopDown metric events can be counting members of 247a sampling read group. Since the SLOTS event must be the leader of a TopDown 248group, the second event of the group is the sampling event. 249For example, perf record -e '{slots, $sampling_event, topdown-retiring}:S' 250 251 252[1] https://software.intel.com/en-us/top-down-microarchitecture-analysis-method-win 253[2] https://github.com/andikleen/pmu-tools/wiki/toplev-manual 254[3] https://software.intel.com/en-us/intel-vtune-amplifier-xe 255[4] https://github.com/andikleen/pmu-tools/tree/master/jevents 256[5] https://sites.google.com/site/analysismethods/yasin-pubs 257
