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

[Steven Rostedt <rostedt@goodmis.org>]

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

>[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