[PATCH] perf db-export: Fix thread__exec_comm()

[Adrian Hunter <adrian.hunter@intel.com>]

Threads synthesized from /proc have comms with a start time of zero, and
not marked as "exec". Currently, there can be 2 such comms. The first is
created by processing a synthesized fork event and is set to the parent's
comm string, and the second by processing a synthesized comm event set to
the thread's current comm string.

In the absence of an "exec" comm, thread__exec_comm() picks the last
(oldest) comm, which, in the case above, is the parent's comm string. For a
main thread, that is very probably wrong. Use the second-to-last in that
case.

This affects only db-export because it is the only user of
thread__exec_comm().

Example:

 $ sudo perf record -a -o pt-a-sleep-1 -e intel_pt//u -- sleep 1
 $ sudo chown ahunter pt-a-sleep-1

 Before:

 $ perf script -i pt-a-sleep-1 --itrace=bep -s tools/perf/scripts/python/export-to-sqlite.py pt-a-sleep-1.db branches calls
 $ sqlite3 -header -column pt-a-sleep-1.db 'select * from comm_threads_view'
 comm_id     command     thread_id   pid         tid
 ----------  ----------  ----------  ----------  ----------
 1           swapper     1           0           0
 2           rcu_sched   2           10          10
 3           kthreadd    3           78          78
 5           sudo        4           15180       15180
 5           sudo        5           15180       15182
 7           kworker/4:  6           10335       10335
 8           kthreadd    7           55          55
 10          systemd     8           865         865
 10          systemd     9           865         875
 13          perf        10          15181       15181
 15          sleep       10          15181       15181
 16          kworker/3:  11          14179       14179
 17          kthreadd    12          29376       29376
 19          systemd     13          746         746
 21          systemd     14          401         401
 23          systemd     15          879         879
 23          systemd     16          879         945
 25          kthreadd    17          556         556
 27          kworker/u1  18          14136       14136
 28          kworker/u1  19          15021       15021
 29          kthreadd    20          509         509
 31          systemd     21          836         836
 31          systemd     22          836         967
 33          systemd     23          1148        1148
 33          systemd     24          1148        1163
 35          kworker/2:  25          17988       17988
 36          kworker/0:  26          13478       13478

 After:

 $ perf script -i pt-a-sleep-1 --itrace=bep -s tools/perf/scripts/python/export-to-sqlite.py pt-a-sleep-1b.db branches calls
 $ sqlite3 -header -column pt-a-sleep-1b.db 'select * from comm_threads_view'
 comm_id     command     thread_id   pid         tid
 ----------  ----------  ----------  ----------  ----------
 1           swapper     1           0           0
 2           rcu_sched   2           10          10
 3           kswapd0     3           78          78
 4           perf        4           15180       15180
 4           perf        5           15180       15182
 6           kworker/4:  6           10335       10335
 7           kcompactd0  7           55          55
 8           accounts-d  8           865         865
 8           accounts-d  9           865         875
 10          perf        10          15181       15181
 12          sleep       10          15181       15181
 13          kworker/3:  11          14179       14179
 14          kworker/1:  12          29376       29376
 15          haveged     13          746         746
 16          systemd-jo  14          401         401
 17          NetworkMan  15          879         879
 17          NetworkMan  16          879         945
 19          irq/131-iw  17          556         556
 20          kworker/u1  18          14136       14136
 21          kworker/u1  19          15021       15021
 22          kworker/u1  20          509         509
 23          thermald    21          836         836
 23          thermald    22          836         967
 25          unity-sett  23          1148        1148
 25          unity-sett  24          1148        1163
 27          kworker/2:  25          17988       17988
 28          kworker/0:  26          13478       13478

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 65de51f93ebf ("perf tools: Identify which comms are from exec")
Cc: stable@vger.kernel.org
---
 tools/perf/util/thread.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/thread.c b/tools/perf/util/thread.c
index 873ab505ca80..590793cc5142 100644
--- a/tools/perf/util/thread.c
+++ b/tools/perf/util/thread.c
@@ -214,14 +214,24 @@ struct comm *thread__comm(const struct thread *thread)
 
 struct comm *thread__exec_comm(const struct thread *thread)
 {
-	struct comm *comm, *last = NULL;
+	struct comm *comm, *last = NULL, *second_last = NULL;
 
 	list_for_each_entry(comm, &thread->comm_list, list) {
 		if (comm->exec)
 			return comm;
+		second_last = last;
 		last = comm;
 	}
 
+	/*
+	 * 'last' with no start time might be the parent's comm of a synthesized
+	 * thread (created by processing a synthesized fork event). For a main
+	 * thread, that is very probably wrong. Prefer a later comm to avoid
+	 * that case.
+	 */
+	if (second_last && !last->start && thread->pid_ == thread->tid)
+		return second_last;
+
 	return last;
 }
 
-- 
2.17.1