Re: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder' for better naming

[Wangnan (F)]

On 2017/11/1 22:22, Liang, Kan wrote:
> > 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.

Then it introduces a main channel switching mechanism, and we need
checking evlist->overwrite and another factor to determine which
one is the main channel. Make things more complex.

> /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.

Yes.

> Thanks,
> Kan