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From:   "Lubashev, Igor" <ilubashe@akamai.com>
To:     Jiri Olsa <jolsa@redhat.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>
CC:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 2/3] perf kvm: Allow running without stdin
Thread-Topic: [PATCH 2/3] perf kvm: Allow running without stdin
Thread-Index: AQHViUT4jEXDkFlf70GojsikrAjFJadoXn2AgBlmKCA=
Date:   Fri, 8 Nov 2019 19:55:14 +0000
Message-ID: <1cf0b4cbb0d04ef2a323faa0dfca0668@ustx2ex-dag1mb5.msg.corp.akamai.com>
References: <1571795693-23558-1-git-send-email-ilubashe@akamai.com>
 <1571795693-23558-3-git-send-email-ilubashe@akamai.com>
 <20191023104245.GL22919@krava>
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Sorry, Jiri!  I replied previously to one comment and totally missed anothe=
r comment above.  Mea culpa.  Not intentional.  Comments inline.

Arnaldo, were you waiting for this comment to take this and the previous pa=
tch in the series?  (I only see that PATCH 3/3 made it to your tree.)


> On Wed, Oct 23, 2019 at 6:43 AM, Jiri Olsa <jolsa@redhat.com> wrote:
>=20
> On Tue, Oct 22, 2019 at 09:54:52PM -0400, Igor Lubashev wrote:
> > Allow perf kvm --stdio to run without access to stdin.
> > This lets perf kvm to run in a batch mode until interrupted.
> >
> > The following now works as expected:
> >
> >   $ perf kvm top --stdio < /dev/null
> >
> > Signed-off-by: Igor Lubashev <ilubashe@akamai.com>
> > ---
> >  tools/perf/builtin-kvm.c | 33 ++++++++++++++++++++-------------
> >  1 file changed, 20 insertions(+), 13 deletions(-)
> >
> > diff --git a/tools/perf/builtin-kvm.c b/tools/perf/builtin-kvm.c index
> > 858da896b518..5217aa3596c7 100644
> > --- a/tools/perf/builtin-kvm.c
> > +++ b/tools/perf/builtin-kvm.c
> > @@ -930,18 +930,20 @@ static int fd_set_nonblock(int fd)
> >
> >  static int perf_kvm__handle_stdin(void)  {
> > -	int c;
> > -
> > -	c =3D getc(stdin);
> > -	if (c =3D=3D 'q')
> > +	switch (getc(stdin)) {
> > +	case 'q':
> > +		done =3D 1;
> >  		return 1;
> > -
> > -	return 0;
> > +	case EOF:
> > +		return 0;
> > +	default:
> > +		return 1;
> > +	}
> >  }
> >
> >  static int kvm_events_live_report(struct perf_kvm_stat *kvm)  {
> > -	int nr_stdin, ret, err =3D -EINVAL;
> > +	int nr_stdin =3D -1, ret, err =3D -EINVAL;
> >  	struct termios save;
> >
> >  	/* live flag must be set first */
> > @@ -972,13 +974,16 @@ static int kvm_events_live_report(struct
> perf_kvm_stat *kvm)
> >  	if (evlist__add_pollfd(kvm->evlist, kvm->timerfd) < 0)
> >  		goto out;
> >
> > -	nr_stdin =3D evlist__add_pollfd(kvm->evlist, fileno(stdin));
> > -	if (nr_stdin < 0)
> > -		goto out;
> > -
> >  	if (fd_set_nonblock(fileno(stdin)) !=3D 0)
> >  		goto out;
> >
> > +	/* add stdin, if it is connected */
> > +	if (getc(stdin) !=3D EOF) {
> > +		nr_stdin =3D evlist__add_pollfd(kvm->evlist, fileno(stdin));
> > +		if (nr_stdin < 0)
> > +			goto out;
> > +	}
> > +
> >  	/* everything is good - enable the events and process */
> >  	evlist__enable(kvm->evlist);
> >
> > @@ -994,8 +999,10 @@ static int kvm_events_live_report(struct
> perf_kvm_stat *kvm)
> >  		if (err)
> >  			goto out;
> >
> > -		if (fda->entries[nr_stdin].revents & POLLIN)
> > -			done =3D perf_kvm__handle_stdin();
> > +		if (nr_stdin >=3D 0 && fda->entries[nr_stdin].revents & POLLIN) {
> > +			if (!perf_kvm__handle_stdin())
>=20
> can this return 0 ? if stdin is EOF then nr_stdin stays -1

The purpose of this check is to catch a case when stdin does not start out =
closed but gets closed later.
Something like:

	sleep 5 | perf kvm ......

Arguably I could had handled both cases in a single uniform way (just this =
code without the initial check).  The effect of this would be one unexpecte=
d cycle of output at the very beginning in the common case (stdin is closed=
 from the start).  So I wanted to make the common case behave well.


> > +				fda->entries[nr_stdin].events =3D 0;
>=20
> why do you need to set events to 0 in here?

As I mentioned before, this is to ensure that no more wakeups are going to =
happen due to stdin getting closed during the perf run.  W/o this, stdin fd=
 would always stay "ready for POLLIN", and perf would never sleep.  (The cl=
eanest way would be to remove stdin fd from the kvm->evlist, but there is n=
o such interface for evlist.  Writing that interface just for this case see=
med imprudent, when events=3D0 would do, and we already access revents abov=
e.)

> thanks,
> jirka

Best,

- Igor