Re: [PATCH] perf stat: force error in fallback on :k events

[Jiri Olsa]

On Mon, Apr 13, 2020 at 04:55:15PM -0700, Ian Rogers wrote:
> From: Stephane Eranian <eranian@xxxxxxxxxx>
> 
> When it is not possible for a non-privilege perf command
> to monitor at the kernel level (:k), the fallback code forces
> a :u. That works if the event was previously monitoring both levels.
> But if the event was already constrained to kernel only, then it does
> not make sense to restrict it to user only.
> Given the code works by exclusion, a kernel only event would have:
> attr->exclude_user = 1
> The fallback code would add:
> attr->exclude_kernel = 1;
> 
> In the end the end would not monitor in either the user level or kernel
> level. In other words, it would count nothing.
> 
> An event programmed to monitor kernel only cannot be switched to user only
> without seriously warning the user.
> 
> This patch forces an error in this case to make it clear the request
> cannot really be satisfied.
> 
> Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
> Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/perf/util/evsel.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index d23db6755f51..d1e8862b86ce 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -2446,6 +2446,13 @@ bool perf_evsel__fallback(struct evsel *evsel, int err,
>  		char *new_name;
>  		const char *sep = ":";
>  
> +		if (evsel->core.attr.exclude_user) {
> +			scnprintf(msg, msgsize,
> +"kernel.perf_event_paranoid=%d, event set to exclude user, so cannot also exclude kernel",
> +				paranoid);
> +			return false;

I'm not able to get this error printed, it seems to be
overwritten by perf_evsel__open_strerror call

please include perf example with the new output

thanks,
jirka

> +		}
> +
>  		/* Is there already the separator in the name. */
>  		if (strchr(name, '/') ||
>  		    strchr(name, ':'))
> -- 
> 2.26.0.110.g2183baf09c-goog
>