Re: [PATCH] perf ftrace latency: Do not read trace files when BPF is used

[Ian Rogers]

On Wed, Aug 26, 2026 at 11:58 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> I've realized that it didn't set up the tracing files when BPF is used
> so poll() just returns immediately.  It ends up with calling poll()
> unnecessarily in a loop.
>
> BPF still needs the loop to wait for the target process exiting or a
> signal from users.  Let's sleep for 1 msec and check that.
>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  tools/perf/builtin-ftrace.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> index 4f881a40c311ae52..e61b9de11e1e41c7 100644
> --- a/tools/perf/builtin-ftrace.c
> +++ b/tools/perf/builtin-ftrace.c
> @@ -1146,6 +1146,11 @@ static int __cmd_latency(struct perf_ftrace *ftrace)
>
>         line[0] = '\0';
>         while (!done) {
> +               if (ftrace->target.use_bpf) {
> +                       usleep(1000);
> +                       continue;
> +               }
> +

To avoid the busy waiting could we:
```
// Globaly:
sem_t sig_sem;
...
// Prior to setting up the signal handler:
sem_init(&sig_sem, 0, 0);
...
// Here:
if (ftrace->target.use_bpf) {
   sem_wait(&sig_sem);
} else {
   while (!done) {
...
}
// In the signal handler:
if (ftrace->target.use_bpf) {
   sem_post(&sig_sem);
}
...
// After removing the signal handler:
sem_destroy(&sig_sem);
```

Thanks,
Ian

>                 if (poll(&pollfd, 1, -1) < 0)
>                         break;
>
> --
> 2.55.0.897.gb25b4bd76c-goog
>