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Date:   Mon, 18 Dec 2017 19:13:06 +0000
From:   "Liang, Kan" <kan.liang@...el.com>
To:     "Wangnan (F)" <wangnan0@...wei.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "jolsa@...nel.org" <jolsa@...nel.org>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "yao.jin@...ux.intel.com" <yao.jin@...ux.intel.com>
Subject: RE: [PATCH V2 6/8] perf top: add overwrite fall back

> 
> On 2017/12/7 7:33, kan.liang@...el.com wrote:
> > From: Kan Liang <kan.liang@...el.com>
> >
> > 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@...el.com>
> > ---
> >   tools/perf/builtin-top.c | 35 +++++++++++++++++++++++++++++++++++
> >   1 file changed, 35 insertions(+)
> >
> > diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> > index 5e15d27..7c462d6 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;
> 
> You can check perf_missing_features.write_backward here. Only do the
> fallback
> when we know the problem is caused by missing of write_backward.
>

perf_missing_features is only defined in evsel.c
I will add an inline function in evsel.c to do the check.

Thanks,
Kan
 
> > +}
> > +
> >   static int perf_top__start_counters(struct perf_top *top)
> >   {
> >   	char msg[BUFSIZ];
> > @@ -954,6 +975,20 @@ static int perf_top__start_counters(struct
> perf_top *top)
> >   try_again:
> >   		if (perf_evsel__open(counter, top->evlist->cpus,
> >   				     top->evlist->threads) < 0) {
> > +
> > +			/*
> > +			 * Specially handle overwrite fall back.
> > +			 * Because perf top is the only tool which has
> > +			 * overwrite mode by default, support
> > +			 * both overwrite and non-overwrite mode, and
> > +			 * require consistent mode for all events.
> > +			 *
> > +			 * May move it to generic code with more tools
> > +			 * have similar attribute.
> > +			 */
> > +			if (perf_top_overwrite_fallback(top, counter))
> > +				goto try_again;
> > +
> 
> It will unconditionally remove backward bit even if the problem
> is caused by other missing feature.
> 
> Thank you.

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