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

[Ian Rogers]

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