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b02027776a
The `perf stat --bpf-counters test` fails intermittently on hybrid
architectures or systems with dynamic frequency scaling (DVFS). This
happens because the test workload (`sqrtloop`) runs for a fixed 1-second
duration, and the CPU frequency can scale dynamically between idle and
maximum frequency. As the first run runs on a cold CPU and the second run
runs on a warmed-up CPU (or vice versa), the number of instructions
executed in 1 second differs by up to 2.2x, violating the comparison
tolerance.
Also, when running as root, BPF tracepoints and scheduling programs
trigger frequently. Since standard `perf stat -e instructions` measures
both user and kernel space instructions, it counts BPF helper and program
execution overheads, whereas the BPF counters themselves do not self-
measure. This introduces a large kernel-space instruction count
discrepancy between standard and BPF counters.
Fix these issues by:
1. Switching the workload to a strictly deterministic, iteration-based
workload: `awk 'BEGIN { for (i=0; i<10000000; i++) sum+=i }'`. We pin
the
workload to a single random allowed CPU using `taskset -c $CPU` via a
bash array.
2. Restricting the counted event to user-space only (`instructions:u` or
`/u`).
3. Tightening the comparison tolerance from 20% to 15%.
These modifications isolate the measurements to user-space instructions of
the deterministic loop, which executes a virtually identical number of
instructions on both runs (with less than 0.001% variation), eliminating
Dynamic Frequency Scaling (DVFS), kernel scheduling noise, and BPF helper
self-measurement overheads.
Fixes: 2c0cb9f560 ("perf test: Add a shell test for 'perf stat --bpf-counters' new option")
Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
95 lines
2.7 KiB
Bash
Executable File
95 lines
2.7 KiB
Bash
Executable File
#!/bin/bash
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# perf stat --bpf-counters test (exclusive)
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# SPDX-License-Identifier: GPL-2.0
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set -e
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# Get the first allowed CPU
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CPU=$(taskset -c -p $$ | awk -F': ' '{print $2}' | awk -F'[,-]' '{print $1}')
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if [ -z "$CPU" ]; then
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CPU=0
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fi
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workload=(taskset -c "$CPU" awk 'BEGIN { for (i=0; i<10000000; i++) sum+=i }')
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# check whether $2 is within +/- 15% of $1
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compare_number()
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{
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first_num=$1
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second_num=$2
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# upper bound is first_num * 115%
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upper=$(expr $first_num + $first_num / 20 \* 3 )
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# lower bound is first_num * 85%
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lower=$(expr $first_num - $first_num / 20 \* 3 )
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if [ $second_num -gt $upper ] || [ $second_num -lt $lower ]; then
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echo "The difference between $first_num and $second_num are greater than 15%."
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exit 1
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fi
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}
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check_counts()
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{
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base_instructions=$1
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bpf_instructions=$2
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if [ "$base_instructions" = "<not" ]; then
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echo "Skipping: instructions event not counted"
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exit 2
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fi
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if [ "$bpf_instructions" = "<not" ]; then
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echo "Failed: instructions not counted with --bpf-counters"
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exit 1
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fi
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}
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test_bpf_counters()
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{
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printf "Testing --bpf-counters "
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base_instructions=$(perf stat --no-big-num -e instructions:u -- "${workload[@]}" 2>&1 | \
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awk -v i=0 -v c=0 '/instructions/ { \
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if ($1 != "<not") { i++; c += $1 } \
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} END { if (i > 0) printf "%.0f", c; else print "<not" }')
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bpf_instructions=$(perf stat --no-big-num --bpf-counters -e instructions:u \
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-- "${workload[@]}" 2>&1 | \
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awk -v i=0 -v c=0 '/instructions/ { \
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if ($1 != "<not") { i++; c += $1 } \
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} END { if (i > 0) printf "%.0f", c; else print "<not" }')
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check_counts $base_instructions $bpf_instructions
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compare_number $base_instructions $bpf_instructions
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echo "[Success]"
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}
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test_bpf_modifier()
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{
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printf "Testing bpf event modifier "
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stat_output=$(perf stat --no-big-num \
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-e instructions/name=base_instructions/u,instructions/name=bpf_instructions/bu \
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-- "${workload[@]}" 2>&1)
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base_instructions=$(echo "$stat_output"| \
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awk -v i=0 -v c=0 '/base_instructions/ { \
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if ($1 != "<not") { i++; c += $1 } \
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} END { if (i > 0) printf "%.0f", c; else print "<not" }')
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bpf_instructions=$(echo "$stat_output"| \
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awk -v i=0 -v c=0 '/bpf_instructions/ { \
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if ($1 != "<not") { i++; c += $1 } \
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} END { if (i > 0) printf "%.0f", c; else print "<not" }')
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check_counts $base_instructions $bpf_instructions
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compare_number $base_instructions $bpf_instructions
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echo "[Success]"
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}
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# skip if --bpf-counters is not supported
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if ! perf stat -e instructions --bpf-counters true > /dev/null 2>&1; then
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if [ "$1" = "-v" ]; then
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echo "Skipping: --bpf-counters not supported"
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perf --no-pager stat -e instructions --bpf-counters true || true
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fi
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exit 2
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fi
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test_bpf_counters
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test_bpf_modifier
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exit 0
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