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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:
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|---|---|---|
| .. | ||
| accounting | ||
| arch | ||
| bootconfig | ||
| bpf | ||
| build | ||
| certs | ||
| cgroup | ||
| counter | ||
| crypto | ||
| debugging | ||
| dma | ||
| docs | ||
| firewire | ||
| firmware | ||
| gpio | ||
| hv | ||
| iio | ||
| include | ||
| kvm/kvm_stat | ||
| laptop | ||
| leds | ||
| lib | ||
| memory-model | ||
| mm | ||
| net | ||
| objtool | ||
| pcmcia | ||
| perf | ||
| power | ||
| rcu | ||
| sched | ||
| sched_ext | ||
| scripts | ||
| sound | ||
| spi | ||
| testing | ||
| thermal | ||
| time | ||
| tracing | ||
| unittests | ||
| usb | ||
| verification | ||
| virtio | ||
| wmi | ||
| workqueue | ||
| writeback | ||
| Makefile | ||