Re: [PATCH v2] perf ftrace latency: Do not read trace files when BPF is used

[Ian Rogers]

On Sun, Aug 30, 2026 at 11:39 PM 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 use a semaphore instead.
>
> Suggested-by: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>

Thanks,
Ian

> ---
>  tools/perf/builtin-ftrace.c | 28 ++++++++++++++++++++++------
>  1 file changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
> index 4f881a40c311ae52..6017493ff179758b 100644
> --- a/tools/perf/builtin-ftrace.c
> +++ b/tools/perf/builtin-ftrace.c
> @@ -6,23 +6,23 @@
>   * Copyright (c) 2020  Changbin Du <changbin.du@xxxxxxxxx>, significant enhancement.
>   */
>
> -#include "builtin.h"
> -
> +#include <ctype.h>
>  #include <errno.h>
> -#include <unistd.h>
> -#include <signal.h>
> -#include <stdlib.h>
>  #include <fcntl.h>
>  #include <inttypes.h>
>  #include <math.h>
>  #include <poll.h>
> -#include <ctype.h>
> +#include <semaphore.h>
> +#include <signal.h>
> +#include <stdlib.h>
> +#include <unistd.h>
>  #include <linux/capability.h>
>  #include <linux/err.h>
>  #include <linux/string.h>
>  #include <linux/zalloc.h>
>  #include <sys/stat.h>
>
> +#include "builtin.h"
>  #include "debug.h"
>  #include <subcmd/pager.h>
>  #include <subcmd/parse-options.h>
> @@ -45,6 +45,7 @@
>
>  static volatile sig_atomic_t workload_exec_errno;
>  static volatile sig_atomic_t done;
> +static sem_t sig_sem;
>
>  static struct stats latency_stats;  /* for tracepoints */
>
> @@ -53,6 +54,7 @@ static char tracing_instance[PATH_MAX];       /* Trace instance directory */
>  static void sig_handler(int sig __maybe_unused)
>  {
>         done = true;
> +       sem_post(&sig_sem);
>  }
>
>  /*
> @@ -68,6 +70,7 @@ static void ftrace__workload_exec_failed_signal(int signo __maybe_unused,
>  {
>         workload_exec_errno = info->si_value.sival_int;
>         done = true;
> +       sem_post(&sig_sem);
>  }
>
>  static bool check_ftrace_capable(void)
> @@ -1146,6 +1149,11 @@ static int __cmd_latency(struct perf_ftrace *ftrace)
>
>         line[0] = '\0';
>         while (!done) {
> +               if (ftrace->target.use_bpf) {
> +                       sem_wait(&sig_sem);
> +                       break;
> +               }
> +
>                 if (poll(&pollfd, 1, -1) < 0)
>                         break;
>
> @@ -1852,6 +1860,8 @@ int cmd_ftrace(int argc, const char **argv)
>         INIT_LIST_HEAD(&ftrace.nograph_funcs);
>         INIT_LIST_HEAD(&ftrace.event_pair);
>
> +       sem_init(&sig_sem, 0, 0);
> +
>         signal(SIGINT, sig_handler);
>         signal(SIGUSR1, sig_handler);
>         signal(SIGCHLD, sig_handler);
> @@ -2015,5 +2025,11 @@ int cmd_ftrace(int argc, const char **argv)
>         delete_filter_func(&ftrace.nograph_funcs);
>         delete_filter_func(&ftrace.event_pair);
>
> +       signal(SIGINT, SIG_DFL);
> +       signal(SIGUSR1, SIG_DFL);
> +       signal(SIGCHLD, SIG_DFL);
> +       signal(SIGPIPE, SIG_DFL);
> +
> +       sem_destroy(&sig_sem);
>         return ret;
>  }
> --
> 2.55.0.897.gb25b4bd76c-goog
>