'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@jouet ~]# 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@jouet ~]#

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

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

index e70313fac5a5..0731b0bf9552 100644
@@ -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) {

I didn't test it on old kernel, but I tested it by forcing sys_perf_event_open return -EINVAL when attr->backward is true by following patch:

index 1d8f2bb..3e91c63 100644
@@ -1418,6 +1418,9 @@ retry_open:
                        pr_debug2("sys_perf_event_open: pid %d  cpu %d  

group_fd %d flags %#lx\n",

                                  pid, cpus->map[cpu], group_fd, flags);

+ if (evsel->attr.write_backward)
+ FD(evsel, cpu, thread) = -EINVAL;
+ else
FD(evsel, cpu, thread) =
sys_perf_event_open(&evsel->attr,
pid,
cpus->map[cpu],

'perf test' skips this test.

Let me switch to an old kernel and try again...

Oops... It does FAILED...

Seems the code I use to emulate old kernel is wrong. I should set errno to EINVAL and make FD(...) = -1.

I agree with your patch. Thank you for pointing it out.