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

[Liang, Kan]

> 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.


Thanks,
Kan
 
> jirka
> 
> 
> ---
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index dbd80268de54..89253f4ff9ee 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1683,9 +1683,6 @@ int perf_evsel__open(struct perf_evsel *evsel,
> struct cpu_map *cpus,
>  	int pid = -1, err;
>  	enum { NO_CHANGE, SET_TO_MAX, INCREASED_MAX } set_rlimit =
> NO_CHANGE;
> 
> -	if (perf_missing_features.write_backward && evsel-
> >attr.write_backward)
> -		return -EINVAL;
> -
>  	if (cpus == NULL) {
>  		static struct cpu_map *empty_cpu_map;
> 
> @@ -1725,6 +1722,8 @@ int perf_evsel__open(struct perf_evsel *evsel,
> struct cpu_map *cpus,
>  	}
> 
>  fallback_missing_features:
> +	if (perf_missing_features.write_backward)
> +		evsel->attr.write_backward = false;
>  	if (perf_missing_features.clockid_wrong)
>  		evsel->attr.clockid = CLOCK_MONOTONIC; /* should always
> work */
>  	if (perf_missing_features.clockid) {