Re: 'perf test backward' failing with older kernels, should Skip

[Wangnan (F)]

On 2016/7/1 5:00, Arnaldo Carvalho de Melo wrote:
> 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 ~]#

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:

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 1d8f2bb..3e91c63 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -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.

> 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 ~]#
>
> 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) {