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

[Tengda Wu <wutengda@huaweicloud.com>]

On 2025/7/11 11:22, Steven Rostedt wrote:
> 
> 
> On July 10, 2025 10:48:54 PM EDT, Tengda Wu <wutengda@huaweicloud.com> wrote:
>>
> 
>> 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?
>>
> 
> Only if it doesn't find three different events, in which case the test would fail regardless.
> 
> The awk script exits out as soon as it finds 3: unique events. It won't go forever, even on slower machines.
> 
> -- Steve

Got it, thank you for explaining this. I have no further questions.

-- Tengda

> 
>> [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