Re: [PATCH V3 10/12] perf top: add overwrite fall back

[Jiri Olsa]

On Thu, Jan 11, 2018 at 09:30:20PM +0000, Liang, Kan wrote:
> > On Thu, Dec 21, 2017 at 10:08:52AM -0800, kan.liang@xxxxxxxxx wrote:
> > > From: Kan Liang <kan.liang@xxxxxxxxx>
> > >
> > > Switch to non-overwrite mode if kernel doesnot support overwrite
> > > ringbuffer.
> > >
> > > It's only effect when overwrite mode is supported.
> > > No change to current behavior.
> > >
> > > Signed-off-by: Kan Liang <kan.liang@xxxxxxxxx>
> > > ---
> > >  tools/perf/builtin-top.c | 36 ++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 36 insertions(+)
> > >
> > > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> > > index 4b85e7b..8d19ef7 100644
> > > --- a/tools/perf/builtin-top.c
> > > +++ b/tools/perf/builtin-top.c
> > > @@ -931,6 +931,27 @@ static int perf_top_overwrite_check(struct
> > perf_top *top)
> > >  	return 0;
> > >  }
> > >
> > > +static int perf_top_overwrite_fallback(struct perf_top *top,
> > > +				       struct perf_evsel *evsel)
> > > +{
> > > +	struct record_opts *opts = &top->record_opts;
> > > +	struct perf_evlist *evlist = top->evlist;
> > > +	struct perf_evsel *counter;
> > > +
> > > +	if (!opts->overwrite)
> > > +		return 0;
> > > +
> > > +	/* only fall back when first event fails */
> > > +	if (evsel != perf_evlist__first(evlist))
> > > +		return 0;
> > > +
> > > +	evlist__for_each_entry(evlist, counter)
> > > +		counter->attr.write_backward = false;
> > > +	opts->overwrite = false;
> > > +	ui__warning("fall back to non-overwrite mode\n");
> > > +	return 1;
> > > +}
> > 
> > we already do that for evsel in perf_evsel__open.. could we make
> > this fallback on one place only?
> > 
> > checking the code, why don't we follow the
> > perf_missing_features.write_backward
> > like we do for other features in the perf_evsel__open and set
> > write_backward accordingly, like in attached patch.
> >
> 
> No, the per-event fallback is explicitly disabled in perf_evsel__open.
> You may refer to commit 32a951b4fd6b  ("perf evlist: Drop redundant
> evsel->overwrite indicator)". 
> 
> Now, only perf top supports the whole fallback. So it is specially handled.

ok

jirka