Now that the default perf_events paranoid level is set to 2, a regular user
cannot monitor kernel level activity anymore. As such, with the following
cmdline:
$ perf record -e cycles date
The perf tool first tries cycles:uk but then falls back to cycles:u
as can be seen in the perf report --header-only output:
cmdline : /export/hda3/tmp/perf.tip record -e cycles ls
event : name = cycles:u, , id = { 436186, ... }
This is okay as long as there is way to learn the priv level was changed
internally by the tool.
But consider a similar example:
$ perf record -b -e cycles date
Error:
You may not have permission to collect stats.
Consider tweaking /proc/sys/kernel/perf_event_paranoid,
which controls use of the performance events system by
unprivileged users (without CAP_SYS_ADMIN).
...
Why is that treated differently given that the branch sampling inherits the
priv level of the first event in this case, i.e., cycles:u? It turns out
that the branch sampling code is more picky and also checks exclude_hv.
In the fallback path, perf record is setting exclude_kernel = 1, but it
does not change exclude_hv. This does not seem to match the restriction
imposed by paranoid = 2.
This patch fixes the problem by forcing exclude_hv = 1 in the fallback
for paranoid=2. With this in place:
$ perf record -b -e cycles date
cmdline : /export/hda3/tmp/perf.tip record -b -e cycles ls
event : name = cycles:u, , id = { 436847, ... }
And the command succeeds as expected.
V2 fix a white space.
Signed-off-by: Stephane Eranian <eran...@google.com>
---
tools/perf/util/evsel.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 85825384f9e8..3cbe06fdf7f7 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2811,9 +2811,11 @@ bool perf_evsel__fallback(struct evsel *evsel, int err,
if (evsel->name)
free(evsel->name);
evsel->name = new_name;
- scnprintf(msg, msgsize,
-"kernel.perf_event_paranoid=%d, trying to fall back to excluding kernel
samples", paranoid);
+ scnprintf(msg, msgsize, "kernel.perf_event_paranoid=%d, trying "
+ "to fall back to excluding kernel and hypervisor "
+ " samples", paranoid);
evsel->core.attr.exclude_kernel = 1;
+ evsel->core.attr.exclude_hv = 1;
return true;
}
--
2.23.0.187.g17f5b7556c-goog
