> On 2017/11/1 21:26, Liang, Kan wrote: > >> The meaning of perf record's "overwrite" option and many "overwrite" > >> in source code are not clear. In perf's code, the 'overwrite' has 2 > >> meanings: > >> 1. Make ringbuffer readonly (perf_evlist__mmap_ex's argument). > >> 2. Set evsel's "backward" attribute (in apply_config_terms). > >> > >> perf record doesn't use meaning 1 at all, but have a overwrite > >> option, its real meaning is setting backward. > >> > > I don't understand here. > > 'overwrite' has 2 meanings. perf record only support 1. > > It should be a bug, and need to be fixed. > > Not a bug, but ambiguous. > > Perf record doesn't need overwrite main channel (we have two channels: > evlist->mmap is main channel and evlist->backward_mmap is backward > evlist->channel), > but some testcases require it, and your new patchset may require it. > 'perf record --overwrite' doesn't set main channel overwrite. What it does is > moving all evsels to backward channel, and we can move some evsels back to > the main channel by /no-overwrite/ setting. This behavior is hard to > understand. >
As my understanding, the 'main channel' should depends on what user sets. If --overwrite is applied, then evlist->backward_mmap should be the 'main channel'. evlist->overwrite should be set to true as well. /no-overwrite/ setting is per-event setting. Only when we finish the global setting, then the per-event setting will be considered. You may refer to apply_config_terms. Thanks, Kan
