linux-kernel - 'perf test backward' failing with older kernels, should Skip

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Date:	Thu, 30 Jun 2016 18:00:52 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	He Kuang <hekuang@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: 'perf test backward' failing with older kernels, should Skip

Hi Wang,

	I booted a machine with the f24 distro kernel and the backward
ring buffer test started FAILing, when it should've noticed that the
kernel doesn't support it and Skip instead:

[root@...et ~]# perf test -v backward
45: Test backward reading from ring buffer                   :
--- start ---
<SNIP>
mmap size 1052672B
Unexpected counter: sample_count=0, comm_count=0
---- end ----
Test backward reading from ring buffer: FAILED!
[root@...et ~]#


Ok, so I think the patch below should do the trick, is that right? With it
I get:

[root@...et ~]# perf test backward
45: Test backward reading from ring buffer                   : Skip
[root@...et ~]#

diff --git a/tools/perf/tests/backward-ring-buffer.c b/tools/perf/tests/backward-ring-buffer.c
index e70313fac5a5..0731b0bf9552 100644
--- a/tools/perf/tests/backward-ring-buffer.c
+++ b/tools/perf/tests/backward-ring-buffer.c
@@ -118,8 +118,10 @@ int test__backward_ring_buffer(int subtest __maybe_unused)
 	perf_evlist__config(evlist, &opts, NULL);
 
 	/* Set backward bit, ring buffer should be writing from end */
-	evlist__for_each_entry(evlist, evsel)
+	evlist__for_each_entry(evlist, evsel) {
+		evsel->overwrite = true;
 		evsel->attr.write_backward = 1;
+	}
 
 	err = perf_evlist__open(evlist);
 	if (err < 0) {