[perf] Update readme with profiling instructions.

perf/README.md

@@ -1,3 +1,5 @@
+# Building and Running
+
 Benchmarks are implemented using [Google Benchmark](https://github.com/google/benchmark).
 
 To build the benchmarks in this directory you need to set the benchmark
@@ -22,3 +24,22 @@ Finally, to run one of the benchmarks:
 It's possible to filter the benchmarks being run and customize the output
 via flags to the benchmark binary. See the
 [Google Benchmark User Guide](https://github.com/google/benchmark/blob/main/docs/user_guide.md#user-guide) for more details.
+
+# Profiling
+
+Configure the build to include debug information for profiling:
+
+```
+CXXFLAGS="-fno-omit-frame-pointer" meson --reconfigure build -Dbenchmark=enabled --buildtype=debug
+```
+
+Then run the benchmark with perf:
+
+```
+perf record -g -o ~/tmp/profiles/subset.prof ./build/perf/benchmark-subset --benchmark_filter="BM_subset_codepoints/subset_notocjk/100000" --benchmark_repetitions=5
+```
+You probably want to filter to a specific benchmark of interest and set the number of repititions high enough to get a good sampling of profile data.
+
+Finally view the profile with:
+
+perf report -i ~/tmp/profiles/subset.prof