perf: sample after exit loses thread correlation - v4

Occassionally events (e.g., context-switch, sched tracepoints) are losing the conversion of sample data associated with a thread. For example:

$ 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 - a 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

v4: per Arnaldo's request add dead flag to thread struct and set when task exits

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 <[email protected]>

Acked-by Adrian Hunter <[email protected]>

Cc Frederic Weisbecker <[email protected]>

Cc Ingo Molnar <[email protected]>

Cc Jiri Olsa <[email protected]>

Cc Mike Galbraith <[email protected]>

Cc Namhyung Kim <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Stephane Eranian <[email protected]>

Cc Adrian Hunter <[email protected]>

---
  tools/perf/util/machine.c |   34 ++++++++++++++++++++--------------
  tools/perf/util/thread.h  |    5 +++++
  2 files changed, 25 insertions(+), 14 deletions(-)

tools/perf/util/machine.c +20 -14

index 4514e7e..574feb7 100644
@@ -1031,11 +1031,27 @@ 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);
 

@@ -1048,18 +1064,8 @@ 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);
 

@@ -1067,7 +1073,7 @@ int machine__process_exit_event(struct machine *machine, union perf_event *event
  		perf_event__fprintf_task(event, stdout);
 
  	if (thread != NULL)
-		machine__remove_thread(machine, thread);
+		thread__exited(thread);
 
  	return 0;
  }

tools/perf/util/thread.h +5 -0

index 13c62c9..32d0601 100644
@@ -16,6 +16,7 @@ struct thread {
  	pid_t			ppid;
  	char			shortname[3];
  	bool			comm_set;
+	bool			dead; /* if set thread has exited */
  	char			*comm;
  	int			comm_len;
 

@@ -26,6 +27,10 @@ struct machine;
 
  struct thread *thread__new(pid_t tid);
  void thread__delete(struct thread *self);
+static inline void thread__exited(struct thread *thread)
+{
+	thread->dead = true;
+}
 
  int thread__set_comm(struct thread *self, const char *comm);
  int thread__comm_len(struct thread *self);