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Date: Fri, 28 Aug 2026 09:59:54 -0700
From: Namhyung Kim <namhyung@kernel.org>
To: Ian Rogers <irogers@google.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	James Clark <james.clark@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	linux-perf-users@vger.kernel.org,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH] perf ftrace latency: Do not read trace files when BPF is
 used
Message-ID: <apG-ij-tYyK9cBSj@google.com>
References: <20260826185815.280451-1-namhyung@kernel.org>
 <CAP-5=fVfmWQaE45Y=pA1rE5JxQLKhfN82+yfRqQE7Hjrb8=aOQ@mail.gmail.com>
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In-Reply-To: <CAP-5=fVfmWQaE45Y=pA1rE5JxQLKhfN82+yfRqQE7Hjrb8=aOQ@mail.gmail.com>

Hi Ian,

On Wed, Aug 26, 2026 at 02:36:27PM -0700, Ian Rogers wrote:
> On Wed, Aug 26, 2026 at 11:58 AM Namhyung Kim <namhyung@kernel.org> 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@goodmis.org>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> > ---
> >  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);
> ```

Looks better, will check.

Thanks,
Namhyung