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Diffstat (limited to '')
| -rw-r--r-- | tools/perf/Documentation/topdown.txt | 162 |
1 files changed, 119 insertions, 43 deletions
diff --git a/tools/perf/Documentation/topdown.txt b/tools/perf/Documentation/topdown.txt index 3c39bb3dc5fa..ae0aee86844f 100644 --- a/tools/perf/Documentation/topdown.txt +++ b/tools/perf/Documentation/topdown.txt @@ -1,46 +1,35 @@ -Using TopDown metrics in user space ------------------------------------ +Using TopDown metrics +--------------------- -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. +TopDown metrics break apart performance bottlenecks. Starting at level +1 it is typical to get metrics on retiring, bad speculation, frontend +bound, and backend bound. Higher levels provide more detail in to the +level 1 bottlenecks, such as at level 2: core bound, memory bound, +heavy operations, light operations, branch mispredicts, machine +clears, fetch latency and fetch bandwidth. For more details see [1][2][3]. -For more details on Topdown see [1][5] +perf stat --topdown implements this using available metrics that vary +per architecture. -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] +% perf stat -a --topdown -I1000 +# time % tma_retiring % tma_backend_bound % tma_frontend_bound % tma_bad_speculation + 1.001141351 11.5 34.9 46.9 6.7 + 2.006141972 13.4 28.1 50.4 8.1 + 3.010162040 12.9 28.1 51.1 8.0 + 4.014009311 12.5 28.6 51.8 7.2 + 5.017838554 11.8 33.0 48.0 7.2 + 5.704818971 14.0 27.5 51.3 7.3 +... -New Topdown features in Ice Lake -=============================== +New Topdown features in Intel 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 retiring bad speculation frontend bound backend bound - 1.001281330 23.0% 15.3% 29.6% 32.1% - 2.003009005 5.0% 6.8% 46.6% 41.6% - 3.004646182 6.7% 6.7% 46.0% 40.6% - 4.006326375 5.0% 6.4% 47.6% 41.0% - 5.007991804 5.1% 6.3% 46.3% 42.3% - 6.009626773 6.2% 7.1% 47.3% 39.3% - 7.011296356 4.7% 6.7% 46.2% 42.4% - 8.012951831 4.7% 6.7% 47.5% 41.1% -... - -This also enables measuring TopDown per thread/process instead -of only per core. - -Using TopDown through RDPMC in applications on Ice Lake -====================================================== +Using TopDown through RDPMC in applications on Intel Ice Lake +============================================================= For more fine grained measurements it can be useful to access the new directly from user space. This is more complicated, @@ -72,6 +61,7 @@ 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 +85,11 @@ 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 +105,14 @@ 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: @@ -121,7 +124,7 @@ to read slots and the topdown metrics at different points of the program: #define RDPMC_METRIC (1 << 29) /* return metric counters */ #define FIXED_COUNTER_SLOTS 3 -#define METRIC_COUNTER_TOPDOWN_L1 0 +#define METRIC_COUNTER_TOPDOWN_L1_L2 0 static inline uint64_t read_slots(void) { @@ -130,7 +133,7 @@ static inline uint64_t read_slots(void) static inline uint64_t read_metrics(void) { - return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1); + return _rdpmc(RDPMC_METRIC | METRIC_COUNTER_TOPDOWN_L1_L2); } Then the program can be instrumented to read these metrics at different @@ -141,6 +144,10 @@ 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 ======================= @@ -152,11 +159,21 @@ The binary ratios in the metric value can be converted to float ratios: #define GET_METRIC(m, i) (((m) >> (i*8)) & 0xff) +/* L1 Topdown metric events */ #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) +/* + * L2 Topdown metric events. + * Available on Sapphire Rapids and later platforms. + */ +#define TOPDOWN_HEAVY_OPS(val) ((float)GET_METRIC(val, 4) / 0xff) +#define TOPDOWN_BR_MISPREDICT(val) ((float)GET_METRIC(val, 5) / 0xff) +#define TOPDOWN_FETCH_LAT(val) ((float)GET_METRIC(val, 6) / 0xff) +#define TOPDOWN_MEM_BOUND(val) ((float)GET_METRIC(val, 7) / 0xff) + and then converted to percent for printing. The ratios in the metric accumulate for the time when the counter @@ -190,8 +207,8 @@ for that time period. 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. +Later the individual ratios of L1 metric events for the measurement period can +be recreated from these counts. slots_delta = slots_b - slots_a retiring_ratio = (float)retiring_slots / slots_delta @@ -205,6 +222,48 @@ from these counts. fe_bound_ratio * 100., be_bound_ratio * 100.); +The individual ratios of L2 metric events for the measurement period can be +recreated from L1 and L2 metric counters. (Available on Sapphire Rapids and +later platforms) + + # compute scaled metrics for measurement a + heavy_ops_slots_a = GET_METRIC(metric_a, 4) * slots_a + br_mispredict_slots_a = GET_METRIC(metric_a, 5) * slots_a + fetch_lat_slots_a = GET_METRIC(metric_a, 6) * slots_a + mem_bound_slots_a = GET_METRIC(metric_a, 7) * slots_a + + # compute delta scaled metrics between b and a + heavy_ops_slots = GET_METRIC(metric_b, 4) * slots_b - heavy_ops_slots_a + br_mispredict_slots = GET_METRIC(metric_b, 5) * slots_b - br_mispredict_slots_a + fetch_lat_slots = GET_METRIC(metric_b, 6) * slots_b - fetch_lat_slots_a + mem_bound_slots = GET_METRIC(metric_b, 7) * slots_b - mem_bound_slots_a + + slots_delta = slots_b - slots_a + heavy_ops_ratio = (float)heavy_ops_slots / slots_delta + light_ops_ratio = retiring_ratio - heavy_ops_ratio; + + br_mispredict_ratio = (float)br_mispredict_slots / slots_delta + machine_clears_ratio = bad_spec_ratio - br_mispredict_ratio; + + fetch_lat_ratio = (float)fetch_lat_slots / slots_delta + fetch_bw_ratio = fe_bound_ratio - fetch_lat_ratio; + + mem_bound_ratio = (float)mem_bound_slots / slota_delta + core_bound_ratio = be_bound_ratio - mem_bound_ratio; + + printf("Heavy Operations %.2f%% Light Operations %.2f%% " + "Branch Mispredict %.2f%% Machine Clears %.2f%% " + "Fetch Latency %.2f%% Fetch Bandwidth %.2f%% " + "Mem Bound %.2f%% Core Bound %.2f%%\n", + heavy_ops_ratio * 100., + light_ops_ratio * 100., + br_mispredict_ratio * 100., + machine_clears_ratio * 100., + fetch_lat_ratio * 100., + fetch_bw_ratio * 100., + mem_bound_ratio * 100., + core_bound_ratio * 100.); + Resetting metrics counters ========================== @@ -231,8 +290,8 @@ 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 -================== +Limits on Intel Ice Lake +======================== Four pseudo TopDown metric events are exposed for the end-users, topdown-retiring, topdown-bad-spec, topdown-fe-bound and topdown-be-bound. @@ -248,9 +307,26 @@ a sampling read group. Since the SLOTS event must be the leader of a TopDown group, the second event of the group is the sampling event. For example, perf record -e '{slots, $sampling_event, topdown-retiring}:S' +Extension on Intel Sapphire Rapids Server +========================================= +The metrics counter is extended to support TMA method level 2 metrics. +The lower half of the register is the TMA level 1 metrics (legacy). +The upper half is also divided into four 8-bit fields for the new level 2 +metrics. Four more TopDown metric events are exposed for the end-users, +topdown-heavy-ops, topdown-br-mispredict, topdown-fetch-lat and +topdown-mem-bound. + +Each of the new level 2 metrics in the upper half is a subset of the +corresponding level 1 metric in the lower half. Software can deduce the +other four level 2 metrics by subtracting corresponding metrics as below. + + Light_Operations = Retiring - Heavy_Operations + Machine_Clears = Bad_Speculation - Branch_Mispredicts + Fetch_Bandwidth = Frontend_Bound - Fetch_Latency + Core_Bound = Backend_Bound - Memory_Bound + [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 +[2] https://sites.google.com/site/analysismethods/yasin-pubs +[3] https://perf.wiki.kernel.org/index.php/Top-Down_Analysis [4] https://github.com/andikleen/pmu-tools/tree/master/jevents -[5] https://sites.google.com/site/analysismethods/yasin-pubs |