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docs: Add LBR profiling instructions to profiling.md

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Signed-off-by: Rob Bradford <robert.bradford@intel.com>
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Rob Bradford 2022-06-15 15:32:31 +01:00
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docs/profiling.md
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@ -43,3 +43,22 @@ $ perf report -g
``` ```
If profiling with a network device attached either the TAP device must be already created and configured or the profiling must be done as root so that the TAP device can be created. If profiling with a network device attached either the TAP device must be already created and configured or the profiling must be done as root so that the TAP device can be created.
## Userspace only profiling with LBR
The use of LBR (Last Branch Record; available since Haswell) offers lower
overhead if only userspace profiling is required. This lower overhead can allow
a higher frequency of sampling. This also removes the requirement to compile
with custom `RUSTFLAGS` however debug symbols should still be included:
e.g.
```
$ perf record --call-graph lbr --all-user --user-callchains -g target/release/cloud-hypervisor \
--kernel ~/src/linux/vmlinux \
--pmem file=~/workloads/focal.raw \
--cpus boot=1 --memory size=1G \
--cmdline "root=/dev/pmem0p1 console=ttyS0" \
--serial tty --console off \
--api-socket=/tmp/api1
```