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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.
## 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
```