Re: [PATCH -next] selftests/ftrace: Prevent potential failure in subsystem-enable test case

[Tengda Wu <wutengda@huaweicloud.com>]

On 2025/7/11 3:34, Steven Rostedt wrote:
> On Thu, 10 Jul 2025 13:01:34 +0000
> Tengda Wu <wutengda@huaweicloud.com> wrote:
> 
> 
>>
>> Fixes: 1a4ea83a6e67 ("selftests/ftrace: Limit length in subsystem-enable tests")
>> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
>> ---
>>  .../selftests/ftrace/test.d/event/subsystem-enable.tc     | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc b/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc
>> index b7c8f29c09a9..3a28adc7b727 100644
>> --- a/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc
>> +++ b/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc
>> @@ -19,8 +19,8 @@ echo 'sched:*' > set_event
>>  yield
>>  
>>  count=`head -n 100 trace | grep -v ^# | awk '{ print $5 }' | sort -u | wc -l`
>> -if [ $count -lt 3 ]; then
>> -    fail "at least fork, exec and exit events should be recorded"
>> +if [ $count -eq 0 ]; then
>> +    fail "none of scheduler events are recorded"
>>  fi
>>  
>>  do_reset
>> @@ -30,8 +30,8 @@ echo 1 > events/sched/enable
>>  yield
>>  
>>  count=`head -n 100 trace | grep -v ^# | awk '{ print $5 }' | sort -u | wc -l`
>> -if [ $count -lt 3 ]; then
>> -    fail "at least fork, exec and exit events should be recorded"
>> +if [ $count -eq 0 ]; then
>> +    fail "none of scheduler events are recorded"
> 
> So if there's a bug that causes the system enable to only enable a
> single event, this will no longer catch it?
> 
> I rather not let the slide.
> 
> Can you test this to see if this works for you?
> 
> -- Steve
> 
> diff --git a/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc b/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc
> index b7c8f29c09a9..46a9e6d92730 100644
> --- a/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc
> +++ b/tools/testing/selftests/ftrace/test.d/event/subsystem-enable.tc
> @@ -14,11 +14,32 @@ fail() { #msg
>      exit_fail
>  }
>  
> +check_unique() {
> +    cat trace_pipe | grep -v '^#' | awk '
> +	BEGIN { cnt = 0; }
> +	{
> +	    for (i = 0; i < cnt; i++) {
> +		if (event[i] == $5) {
> +		    break;
> +		}
> +	    }
> +	    if (i == cnt) {
> +		event[cnt++] = $5;
> +		if (cnt > 2) {
> +		    exit;
> +		}
> +	    }
> +	}
> +	END {
> +	    printf "%d", cnt;
> +	}'
> +}
> +
>  echo 'sched:*' > set_event
>  
>  yield
>  
> -count=`head -n 100 trace | grep -v ^# | awk '{ print $5 }' | sort -u | wc -l`
> +count=`check_unique`
>  if [ $count -lt 3 ]; then
>      fail "at least fork, exec and exit events should be recorded"
>  fi
> @@ -29,7 +50,7 @@ echo 1 > events/sched/enable
>  
>  yield
>  
> -count=`head -n 100 trace | grep -v ^# | awk '{ print $5 }' | sort -u | wc -l`
> +count=`check_unique`
>  if [ $count -lt 3 ]; then
>      fail "at least fork, exec and exit events should be recorded"
>  fi

The patch works well - after ~50 test iterations, we haven't observed any
recurrence of the test case failures.

However, I'm concerned that using 'cat trace_pipe' (like the original
'cat trace' method) could bring back the stopping problem [1] on slower
systems.

Could a slow trace_pipe reader (slower than sched event generation rate)
reintroduce the original race condition?

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1a4ea83a6e67f1415a1f17c1af5e9c814c882bb5

Some test details:

$ ./ftracetest -vvv subsystem-enable.tc
[...]
+ echo sched:*
+ yield
+ ping 127.0.0.1 -c 1
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.538 ms

--- 127.0.0.1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 1ms
rtt min/avg/max/mdev = 0.538/0.538/0.538/0.000 ms
+ check_unique
+ cat trace_pipe
+ grep -v ^#
+ awk 
        BEGIN { cnt = 0; }
        {
            for (i = 0; i < cnt; i++) {
                if (event[i] == $5) {
                    break;
                }
            }
            if (i == cnt) {
                event[cnt++] = $5;
                if (cnt > 2) {
                    exit;
                }
            }
        }
        END {
            printf "%d", cnt;
        }
+ count=3
+ [ 3 -lt 3 ]
+ do_reset
[...]

Regards,
Tengda