Re: [PATCH V2 6/8] perf top: add overwrite fall back

["Wangnan (F)" <wangnan0@huawei.com>]

On 2017/12/7 7:33, kan.liang@intel.com wrote:
> From: Kan Liang <kan.liang@intel.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@intel.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.

> +}
> +
>   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.