Re: [PATCH 01/19] perf: sample after exit loses thread correlation - v3

Arnaldo Carvalho de Melo
2013-08-08T14:53:54Z

Em Wed, Aug 07, 2013 at 10:50:43PM -0400, David Ahern escreveu:
> Occassionally events (e.g., context-switch, sched tracepoints) are losing
> the conversion of sample data associated with a thread. For example:

Humm, if we have a tool that traverses the list of threads in a machine
it will not know, after one of them exits, about it being dead, so I
think this needs to add a thread->dead, no?

As of now, from what I can remember, the closest to such a tool would
be:

perf top --sort pid

But that uses hist_entries that would eventually be decayed as samples
would cease to be taken at most a few moments after the EXIT event.

But at least for debugging purposes, machine__fprintf() would list dead
threads as being present, i.e. alive till its pid gets reused.

This is the only, minor, problem that I see with this solution, what do
you think?

- Arnaldo

> $ perf record -e sched:sched_switch -c 1 -a -- sleep 5
> $ perf script
> <selected events shown>
> ls 30482 [000] 1379727.583037: sched:sched_switch: prev_comm=ls prev_pid=30482 ...
> ls 30482 [000] 1379727.586339: sched:sched_switch: prev_comm=ls prev_pid=30482 ...
> :30482 30482 [000] 1379727.589462: sched:sched_switch: prev_comm=ls prev_pid=30482 ...
>
> The last line lost the conversion from tid to comm. If you look at the events
> (perf script -D) you see why - SAMPLE event is generated after the EXIT:
>
> 0 1379727589449774 0x1540b0 [0x38]: PERF_RECORD_EXIT(30482:30482):(30482:30482)
> 0 1379727589462497 0x1540e8 [0x80]: PERF_RECORD_SAMPLE(IP, 1): 30482/30482: 0xffffffff816416f1 period: 1 addr: 0
> ... thread: :30482:30482
>
> When perf processes the EXIT event the thread is moved to the dead_threads
> list. When the SAMPLE event is processed no thread exists for the pid so a new
> one is created by machine__findnew_thread.
>
> This patch address the problem by delaying the move to the dead_threads list
> until the tid is re-used (per Adrian's suggestion).
>
> With this patch we get the previous example shows:
>
> ls 30482 [000] 1379727.583037: sched:sched_switch: prev_comm=ls prev_pid=30482 ...
> ls 30482 [000] 1379727.586339: sched:sched_switch: prev_comm=ls prev_pid=30482 ...
> ls 30482 [000] 1379727.589462: sched:sched_switch: prev_comm=ls prev_pid=30482 ...
>
> and
>
> 0 1379727589449774 0x1540b0 [0x38]: PERF_RECORD_EXIT(30482:30482):(30482:30482)
> 0 1379727589462497 0x1540e8 [0x80]: PERF_RECORD_SAMPLE(IP, 1): 30482/30482: 0xffffffff816416f1 period: 1 addr: 0
> ... thread: ls:30482
>
> v3: re-do from a time based check to a delayed move to dead_threads list
>
> v2: Rebased to latest perf/core branch. Changed time comparison to use
> a macro which explicitly shows the time basis
>
> Signed-off-by: David Ahern <dsahern@gmail.com>
> Acked-by: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Mike Galbraith <efault@gmx.de>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephane Eranian <eranian@google.com>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> ---
> tools/perf/util/machine.c | 37 +++++++++++++++++++------------------
> 1 file changed, 19 insertions(+), 18 deletions(-)
>
> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
> index 6fcc358..7784a9d 100644
> --- a/tools/perf/util/machine.c
> +++ b/tools/perf/util/machine.c
> @@ -1011,11 +1011,27 @@ out_problem:
> return 0;
> }
>
> +static void machine__remove_thread(struct machine *machine, struct thread *th)
> +{
> + machine->last_match = NULL;
> + rb_erase(&th->rb_node, &machine->threads);
> + /*
> + * We may have references to this thread, for instance in some hist_entry
> + * instances, so just move them to a separate list.
> + */
> + list_add_tail(&th->node, &machine->dead_threads);
> +}
> +
> int machine__process_fork_event(struct machine *machine, union perf_event *event)
> {
> - struct thread *thread = machine__findnew_thread(machine, event->fork.tid);
> + struct thread *thread = machine__find_thread(machine, event->fork.tid);
> struct thread *parent = machine__findnew_thread(machine, event->fork.ptid);
>
> + /* if a thread currently exists for the thread id remove it */
> + if (thread != NULL)
> + machine__remove_thread(machine, thread);
> +
> + thread = machine__findnew_thread(machine, event->fork.tid);
> if (dump_trace)
> perf_event__fprintf_task(event, stdout);
>
> @@ -1028,27 +1044,12 @@ int machine__process_fork_event(struct machine *machine, union perf_event *event
> return 0;
> }
>
> -static void machine__remove_thread(struct machine *machine, struct thread *th)
> -{
> - machine->last_match = NULL;
> - rb_erase(&th->rb_node, &machine->threads);
> - /*
> - * We may have references to this thread, for instance in some hist_entry
> - * instances, so just move them to a separate list.
> - */
> - list_add_tail(&th->node, &machine->dead_threads);
> -}
> -
> -int machine__process_exit_event(struct machine *machine, union perf_event *event)
> +int machine__process_exit_event(struct machine *machine __maybe_unused,
> + union perf_event *event)
> {
> - struct thread *thread = machine__find_thread(machine, event->fork.tid);
> -
> if (dump_trace)
> perf_event__fprintf_task(event, stdout);
>
> - if (thread != NULL)
> - machine__remove_thread(machine, thread);
> -
> return 0;
> }
>
> --
> 1.7.10.1