Simpleperf: Only one FEAT_FILE2 feature section can be read from the file about ndk HOT 2 CLOSED

Yes, it works as intended. You can use simpleperf dump to dump a perf.data, and check symbols from multiple dsos in FEAT_FILE2 section.

from ndk.

Oh indeed, it does work! The code is just a bit confusing - it looks as if it's trying to write multiple different feature sections, but instead it's all concatenated into one single big feature section.

But after writing each of them, it updates the section offset + size of the FEAT_FILE2 feature to point to the last-written section.

No, it only updates the size. RecordFileWriter::WriteFeatureBegin leaves the offset untouched if the feature already exists in RecordFileWriter::features_.
RecordFileWriter::WriteFeatureEnd updates the size.

So really, I was confused by the repeated calls to WriteFeatureBegin and WriteFeatureEnd. It would be clearer if there was only one call to each of them total, rather than one call per DSO. Repeated calls to WriteFeatureBegin are no-ops, and repeated calls to WriteFeatureEnd overwrite the previous call's effects.

RecordFileReader::LoadBuildIdAndFileFeatures appears to try to read multiple file feature sections. But the offset + size for each section is obtained from feature_section_descriptors_.find(FEAT_FILE2)->second, which will return the same values every time.

That's correct, but the read_pos will be the relative offset to the start of the section, and when RecordFileReader::ReadFileFeature returns, only the data for one DSO has been read, so read_pos will now point partway into the section, to the beginning of the data for the next DSO. So read_pos is different for each call to RecordFileReader::ReadFileFeature. Only once the data for all DSOs has been read will read_pos be equal to desc.size.

Again, what's confusing here is that there are multiple calls to RecordFileReader::ReadFileFeature, even though all calls are reading bytes from the same feature section. And the feature section descriptor is obtained repeatedly on each call even though it's always the same.

from ndk.