1 perf-arm-spe(1) 1 perf-arm-spe(1)
2 ================ 2 ================
3 3
4 NAME 4 NAME
5 ---- 5 ----
6 perf-arm-spe - Support for Arm Statistical Pro 6 perf-arm-spe - Support for Arm Statistical Profiling Extension within Perf tools
7 7
8 SYNOPSIS 8 SYNOPSIS
9 -------- 9 --------
10 [verse] 10 [verse]
11 'perf record' -e arm_spe// 11 'perf record' -e arm_spe//
12 12
13 DESCRIPTION 13 DESCRIPTION
14 ----------- 14 -----------
15 15
16 The SPE (Statistical Profiling Extension) feat 16 The SPE (Statistical Profiling Extension) feature provides accurate attribution of latencies and
17 events down to individual instructions. Rathe 17 events down to individual instructions. Rather than being interrupt-driven, it picks an
18 instruction to sample and then captures data f 18 instruction to sample and then captures data for it during execution. Data includes execution time
19 in cycles. For loads and stores it also includ 19 in cycles. For loads and stores it also includes data address, cache miss events, and data origin.
20 20
21 The sampling has 5 stages: 21 The sampling has 5 stages:
22 22
23 1. Choose an operation 23 1. Choose an operation
24 2. Collect data about the operation 24 2. Collect data about the operation
25 3. Optionally discard the record based on a 25 3. Optionally discard the record based on a filter
26 4. Write the record to memory 26 4. Write the record to memory
27 5. Interrupt when the buffer is full 27 5. Interrupt when the buffer is full
28 28
29 Choose an operation 29 Choose an operation
30 ~~~~~~~~~~~~~~~~~~~ 30 ~~~~~~~~~~~~~~~~~~~
31 31
32 This is chosen from a sample population, for S 32 This is chosen from a sample population, for SPE this is an IMPLEMENTATION DEFINED choice of all
33 architectural instructions or all micro-ops. S 33 architectural instructions or all micro-ops. Sampling happens at a programmable interval. The
34 architecture provides a mechanism for the SPE 34 architecture provides a mechanism for the SPE driver to infer the minimum interval at which it should
35 sample. This minimum interval is used by the d 35 sample. This minimum interval is used by the driver if no interval is specified. A pseudo-random
36 perturbation is also added to the sampling int 36 perturbation is also added to the sampling interval by default.
37 37
38 Collect data about the operation 38 Collect data about the operation
39 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 39 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
40 40
41 Program counter, PMU events, timings and data 41 Program counter, PMU events, timings and data addresses related to the operation are recorded.
42 Sampling ensures there is only one sampled ope 42 Sampling ensures there is only one sampled operation is in flight.
43 43
44 Optionally discard the record based on a filte 44 Optionally discard the record based on a filter
45 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 45 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
46 46
47 Based on programmable criteria, choose whether 47 Based on programmable criteria, choose whether to keep the record or discard it. If the record is
48 discarded then the flow stops here for this sa 48 discarded then the flow stops here for this sample.
49 49
50 Write the record to memory 50 Write the record to memory
51 ~~~~~~~~~~~~~~~~~~~~~~~~~~ 51 ~~~~~~~~~~~~~~~~~~~~~~~~~~
52 52
53 The record is appended to a memory buffer 53 The record is appended to a memory buffer
54 54
55 Interrupt when the buffer is full 55 Interrupt when the buffer is full
56 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 56 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
57 57
58 When the buffer fills, an interrupt is sent an 58 When the buffer fills, an interrupt is sent and the driver signals Perf to collect the records.
59 Perf saves the raw data in the perf.data file. 59 Perf saves the raw data in the perf.data file.
60 60
61 Opening the file 61 Opening the file
62 ---------------- 62 ----------------
63 63
64 Up until this point no decoding of the SPE dat 64 Up until this point no decoding of the SPE data was done by either the kernel or Perf. Only when the
65 recorded file is opened with 'perf report' or 65 recorded file is opened with 'perf report' or 'perf script' does the decoding happen. When decoding
66 the data, Perf generates "synthetic samples" a 66 the data, Perf generates "synthetic samples" as if these were generated at the time of the
67 recording. These samples are the same as if no 67 recording. These samples are the same as if normal sampling was done by Perf without using SPE,
68 although they may have more attributes associa 68 although they may have more attributes associated with them. For example a normal sample may have
69 just the instruction pointer, but an SPE sampl 69 just the instruction pointer, but an SPE sample can have data addresses and latency attributes.
70 70
71 Why Sampling? 71 Why Sampling?
72 ------------- 72 -------------
73 73
74 - Sampling, rather than tracing, cuts down th 74 - Sampling, rather than tracing, cuts down the profiling problem to something more manageable for
75 hardware. Only one sampled operation is in fl 75 hardware. Only one sampled operation is in flight at a time.
76 76
77 - Allows precise attribution data, including: 77 - Allows precise attribution data, including: Full PC of instruction, data virtual and physical
78 addresses. 78 addresses.
79 79
80 - Allows correlation between an instruction a 80 - Allows correlation between an instruction and events, such as TLB and cache miss. (Data source
81 indicates which particular cache was hit, but 81 indicates which particular cache was hit, but the meaning is implementation defined because
82 different implementations can have different 82 different implementations can have different cache configurations.)
83 83
84 However, SPE does not provide any call-graph i 84 However, SPE does not provide any call-graph information, and relies on statistical methods.
85 85
86 Collisions 86 Collisions
87 ---------- 87 ----------
88 88
89 When an operation is sampled while a previous 89 When an operation is sampled while a previous sampled operation has not finished, a collision
90 occurs. The new sample is dropped. Collisions 90 occurs. The new sample is dropped. Collisions affect the integrity of the data, so the sample rate
91 should be set to avoid collisions. 91 should be set to avoid collisions.
92 92
93 The 'sample_collision' PMU event can be used t 93 The 'sample_collision' PMU event can be used to determine the number of lost samples. Although this
94 count is based on collisions _before_ filterin 94 count is based on collisions _before_ filtering occurs. Therefore this can not be used as an exact
95 number for samples dropped that would have mad 95 number for samples dropped that would have made it through the filter, but can be a rough
96 guide. 96 guide.
97 97
98 The effect of microarchitectural sampling 98 The effect of microarchitectural sampling
99 ----------------------------------------- 99 -----------------------------------------
100 100
101 If an implementation samples micro-operations 101 If an implementation samples micro-operations instead of instructions, the results of sampling must
102 be weighted accordingly. 102 be weighted accordingly.
103 103
104 For example, if a given instruction A is alway 104 For example, if a given instruction A is always converted into two micro-operations, A0 and A1, it
105 becomes twice as likely to appear in the sampl 105 becomes twice as likely to appear in the sample population.
106 106
107 The coarse effect of conversions, and, if appl 107 The coarse effect of conversions, and, if applicable, sampling of speculative operations, can be
108 estimated from the 'sample_pop' and 'inst_reti 108 estimated from the 'sample_pop' and 'inst_retired' PMU events.
109 109
110 Kernel Requirements 110 Kernel Requirements
111 ------------------- 111 -------------------
112 112
113 The ARM_SPE_PMU config must be set to build as 113 The ARM_SPE_PMU config must be set to build as either a module or statically.
114 114
115 Depending on CPU model, the kernel may need to 115 Depending on CPU model, the kernel may need to be booted with page table isolation disabled
116 (kpti=off). If KPTI needs to be disabled, this 116 (kpti=off). If KPTI needs to be disabled, this will fail with a console message "profiling buffer
117 inaccessible. Try passing 'kpti=off' on the ke 117 inaccessible. Try passing 'kpti=off' on the kernel command line".
118 118
119 For the full criteria that determine whether K <<
120 unmap_kernel_at_el0() in the kernel sources. C <<
121 are on the CPUs in kpti_safe_list, or on Arm v <<
122 <<
123 The SPE interrupt must also be described by th <<
124 disabled (or isn't required to be disabled) bu <<
125 /sys/bus/event_source/devices/, then it's poss <<
126 ACPI or DT. In this case no warning will be pr <<
127 <<
128 Capturing SPE with perf command-line tools 119 Capturing SPE with perf command-line tools
129 ------------------------------------------ 120 ------------------------------------------
130 121
131 You can record a session with SPE samples: 122 You can record a session with SPE samples:
132 123
133 perf record -e arm_spe// -- ./mybench 124 perf record -e arm_spe// -- ./mybench
134 125
135 The sample period is set from the -c option, a 126 The sample period is set from the -c option, and because the minimum interval is used by default
136 it's recommended to set this to a higher value 127 it's recommended to set this to a higher value. The value is written to PMSIRR.INTERVAL.
137 128
138 Config parameters 129 Config parameters
139 ~~~~~~~~~~~~~~~~~ 130 ~~~~~~~~~~~~~~~~~
140 131
141 These are placed between the // in the event a 132 These are placed between the // in the event and comma separated. For example '-e
142 arm_spe/load_filter=1,min_latency=10/' 133 arm_spe/load_filter=1,min_latency=10/'
143 134
144 branch_filter=1 - collect branches only 135 branch_filter=1 - collect branches only (PMSFCR.B)
145 event_filter=<mask> - filter on specific eve 136 event_filter=<mask> - filter on specific events (PMSEVFR) - see bitfield description below
146 jitter=1 - use jitter to avoid re 137 jitter=1 - use jitter to avoid resonance when sampling (PMSIRR.RND)
147 load_filter=1 - collect loads only (PM 138 load_filter=1 - collect loads only (PMSFCR.LD)
148 min_latency=<n> - collect only samples w 139 min_latency=<n> - collect only samples with this latency or higher* (PMSLATFR)
149 pa_enable=1 - collect physical addre 140 pa_enable=1 - collect physical address (as well as VA) of loads/stores (PMSCR.PA) - requires privilege
150 pct_enable=1 - collect physical times 141 pct_enable=1 - collect physical timestamp instead of virtual timestamp (PMSCR.PCT) - requires privilege
151 store_filter=1 - collect stores only (P 142 store_filter=1 - collect stores only (PMSFCR.ST)
152 ts_enable=1 - enable timestamping wi 143 ts_enable=1 - enable timestamping with value of generic timer (PMSCR.TS)
153 144
154 +++*+++ Latency is the total latency from the 145 +++*+++ Latency is the total latency from the point at which sampling started on that instruction, rather
155 than only the execution latency. 146 than only the execution latency.
156 147
157 Only some events can be filtered on; these inc 148 Only some events can be filtered on; these include:
158 149
159 bit 1 - instruction retired (i.e. omit s 150 bit 1 - instruction retired (i.e. omit speculative instructions)
160 bit 3 - L1D refill 151 bit 3 - L1D refill
161 bit 5 - TLB refill 152 bit 5 - TLB refill
162 bit 7 - mispredict 153 bit 7 - mispredict
163 bit 11 - misaligned access 154 bit 11 - misaligned access
164 155
165 So to sample just retired instructions: 156 So to sample just retired instructions:
166 157
167 perf record -e arm_spe/event_filter=2/ -- ./ 158 perf record -e arm_spe/event_filter=2/ -- ./mybench
168 159
169 or just mispredicted branches: 160 or just mispredicted branches:
170 161
171 perf record -e arm_spe/event_filter=0x80/ -- 162 perf record -e arm_spe/event_filter=0x80/ -- ./mybench
172 163
173 Viewing the data 164 Viewing the data
174 ~~~~~~~~~~~~~~~~~ 165 ~~~~~~~~~~~~~~~~~
175 166
176 By default perf report and perf script will as 167 By default perf report and perf script will assign samples to separate groups depending on the
177 attributes/events of the SPE record. Because i 168 attributes/events of the SPE record. Because instructions can have multiple events associated with
178 them, the samples in these groups are not nece 169 them, the samples in these groups are not necessarily unique. For example perf report shows these
179 groups: 170 groups:
180 171
181 Available samples 172 Available samples
182 0 arm_spe// 173 0 arm_spe//
183 0 dummy:u 174 0 dummy:u
184 21 l1d-miss 175 21 l1d-miss
185 897 l1d-access 176 897 l1d-access
186 5 llc-miss 177 5 llc-miss
187 7 llc-access 178 7 llc-access
188 2 tlb-miss 179 2 tlb-miss
189 1K tlb-access 180 1K tlb-access
190 36 branch-miss 181 36 branch-miss
191 0 remote-access 182 0 remote-access
192 900 memory 183 900 memory
193 184
194 The arm_spe// and dummy:u events are implement 185 The arm_spe// and dummy:u events are implementation details and are expected to be empty.
195 186
196 To get a full list of unique samples that are 187 To get a full list of unique samples that are not sorted into groups, set the itrace option to
197 generate 'instruction' samples. The period opt 188 generate 'instruction' samples. The period option is also taken into account, so set it to 1
198 instruction unless you want to further downsam 189 instruction unless you want to further downsample the already sampled SPE data:
199 190
200 perf report --itrace=i1i 191 perf report --itrace=i1i
201 192
202 Memory access details are also stored on the s 193 Memory access details are also stored on the samples and this can be viewed with:
203 194
204 perf report --mem-mode 195 perf report --mem-mode
205 196
206 Common errors 197 Common errors
207 ~~~~~~~~~~~~~ 198 ~~~~~~~~~~~~~
208 199
209 - "Cannot find PMU `arm_spe'. Missing kernel 200 - "Cannot find PMU `arm_spe'. Missing kernel support?"
210 201
211 Module not built or loaded, KPTI not disabl !! 202 Module not built or loaded, KPTI not disabled (see above), or running on a VM
212 or running on a VM. See 'Kernel Requirement <<
213 203
214 - "Arm SPE CONTEXT packets not found in the t 204 - "Arm SPE CONTEXT packets not found in the traces."
215 205
216 Root privilege is required to collect conte 206 Root privilege is required to collect context packets. But these only increase the accuracy of
217 assigning PIDs to kernel samples. For users 207 assigning PIDs to kernel samples. For userspace sampling this can be ignored.
218 208
219 - Excessively large perf.data file size 209 - Excessively large perf.data file size
220 210
221 Increase sampling interval (see above) 211 Increase sampling interval (see above)
222 212
223 213
224 SEE ALSO 214 SEE ALSO
225 -------- 215 --------
226 216
227 linkperf:perf-record[1], linkperf:perf-script[ 217 linkperf:perf-record[1], linkperf:perf-script[1], linkperf:perf-report[1],
228 linkperf:perf-inject[1] 218 linkperf:perf-inject[1]
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