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

From: Jiri Olsa
Date: Tue Apr 14 2020 - 09:03:12 EST


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
>