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Date: Tue, 19 May 2015 16:46:43 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Jiri Olsa <jolsa@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
David Ahern <dsahern@...il.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Andi Kleen <andi@...stfloor.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 35/40] perf record: Synthesize COMM event for a command
line workload
On Mon, May 18, 2015 at 09:45:35AM -0300, Arnaldo Carvalho de Melo wrote:
> Em Mon, May 18, 2015 at 09:30:50AM +0900, Namhyung Kim escreveu:
> > When perf creates a new child to profile, the events are enabled on
> > exec(). And in this case, it doesn't synthesize any event for the
> > child since they'll be generated during exec(). But there's an window
> > between the enabling and the event generation.
> >
> > It used to be overcome since samples are only in kernel (so we always
> > have the map) and the comm is overridden by a later COMM event.
> > However it won't work anymore since those samples will go to a missing
> > thread now but the COMM event will create a (current) thread. This
> > leads to those early samples (like native_write_msr_safe) not having a
> > comm but pid (like ':15328').
>
> > So it needs to synthesize COMM event for the child explicitly before
> > enabling so that it can have a correct comm. But at this time, the
> > comm will be "perf" since it's not exec-ed yet.
>
> This looks reasonable, but I think it probably needs to be done
> somewhere in perf_evlist__prepare_workload() or
> perf_evlist__start_workload(), as this affects other tools as well, like
> 'top', 'trace' and any other that may want to do this start-workload use
> case.
Hmm.. I need to look at this again as it only affects on processing
indexed data files which used to have a separate missing threads tree.
That's the reason why I didn't put it in a generic place like you
said.
However I changed not to use the separate tree - the purpose of the
tree was to reduce lock acquisition on thread searching but it already
grabs a rwlock with thread refcounting change.
Will check whether this is still needed..
Thanks,
Namhyung
>
> I also wonder if we can't overcome this without using /proc, i.e.
> actually moving the "start the workload" to just before the fork, so
> that the kernel covers that as well.
>
> Or, alternatively, the thread can be created without having to look at
> /proc at all, but by directly creating the struct thread, with the
> correct COMM, pid, etc, that we know, since we forked it, etc.
>
> - Arnaldo
>
> > Signed-off-by: Namhyung Kim <namhyung@...nel.org>
> > ---
> > tools/perf/builtin-record.c | 18 +++++++++++++++++-
> > tools/perf/util/event.c | 2 +-
> > tools/perf/util/event.h | 5 +++++
> > 3 files changed, 23 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> > index 978ebf648aab..153f38e35555 100644
> > --- a/tools/perf/builtin-record.c
> > +++ b/tools/perf/builtin-record.c
> > @@ -766,8 +766,24 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
> > /*
> > * Let the child rip
> > */
> > - if (forks)
> > + if (forks) {
> > + union perf_event *comm_event;
> > +
> > + comm_event = malloc(sizeof(*comm_event) + machine->id_hdr_size);
> > + if (comm_event == NULL)
> > + goto out_child;
> > +
> > + err = perf_event__synthesize_comm(tool, comm_event,
> > + rec->evlist->workload.pid,
> > + process_synthesized_event,
> > + machine);
> > + free(comm_event);
> > +
> > + if (err < 0)
> > + goto out_child;
> > +
> > perf_evlist__start_workload(rec->evlist);
> > + }
> >
> > if (opts->initial_delay) {
> > usleep(opts->initial_delay * 1000);
> > diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
> > index 930d45d5a37a..3adca1302150 100644
> > --- a/tools/perf/util/event.c
> > +++ b/tools/perf/util/event.c
> > @@ -165,7 +165,7 @@ static int perf_event__prepare_comm(union perf_event *event, pid_t pid,
> > return 0;
> > }
> >
> > -static pid_t perf_event__synthesize_comm(struct perf_tool *tool,
> > +pid_t perf_event__synthesize_comm(struct perf_tool *tool,
> > union perf_event *event, pid_t pid,
> > perf_event__handler_t process,
> > struct machine *machine)
> > diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
> > index 40e02544f861..9aacd558ac0f 100644
> > --- a/tools/perf/util/event.h
> > +++ b/tools/perf/util/event.h
> > @@ -452,6 +452,11 @@ int perf_event__synthesize_mmap_events(struct perf_tool *tool,
> > struct machine *machine,
> > bool mmap_data);
> >
> > +pid_t perf_event__synthesize_comm(struct perf_tool *tool,
> > + union perf_event *event, pid_t pid,
> > + perf_event__handler_t process,
> > + struct machine *machine);
> > +
> > size_t perf_event__fprintf_comm(union perf_event *event, FILE *fp);
> > size_t perf_event__fprintf_mmap(union perf_event *event, FILE *fp);
> > size_t perf_event__fprintf_mmap2(union perf_event *event, FILE *fp);
> > --
> > 2.4.0
> --
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