[PATCH] trace: Reorder display of TGID to be after PID

[Joel Fernandes]

From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>

Currently ftrace displays data in trace output like so:

                                       _-----=> irqs-off
                                      / _----=> need-resched
                                     | / _---=> hardirq/softirq
                                     || / _--=> preempt-depth
                                     ||| /     delay
            TASK-PID   CPU    TGID   ||||    TIMESTAMP  FUNCTION
               | |       |      |    ||||       |         |
            bash-1091  [000] ( 1091) d..2    28.313544: sched_switch:

However Android's trace visualization tools expect a slightly different
format due to an out-of-tree patch patch that was been carried for a
decade, notice that the TGID and CPU fields are reversed:

                                       _-----=> irqs-off
                                      / _----=> need-resched
                                     | / _---=> hardirq/softirq
                                     || / _--=> preempt-depth
                                     ||| /     delay
            TASK-PID    TGID   CPU   ||||    TIMESTAMP  FUNCTION
               | |        |      |   ||||       |         |
            bash-1091  ( 1091) [002] d..2    64.965177: sched_switch:

>From kernel v4.13 onwards, during which TGID was introduced, tracing
with systrace on all Android kernels will break (most Android kernels
have been on 4.9 with Android patches, so this issues hasn't been seen
yet). From v4.13 onwards things will break.

The chrome browser's tracing tools also embed the systrace viewer which
uses the legacy TGID format and updates to that are known to be
difficult to make.

Considering this, I suggest we make this change to the upstream kernel
and backport it to all Android kernels. I believe this feature is merged
recently enough into the upstream kernel that it shouldn't be a problem.
Also logically, IMO it makes more sense to group the TGID with the
TASK-PID and the CPU after these.

Fixes: 441dae8f2f29 ("tracing: Add support for display of tgid in trace output")
Cc: jreck@xxxxxxxxxx
Cc: tkjos@xxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 kernel/trace/trace.c        | 8 ++++----
 kernel/trace/trace_output.c | 5 +++--
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index c9336e98ac59..5711ca3ba4c9 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3364,8 +3364,8 @@ static void print_func_help_header(struct trace_buffer *buf, struct seq_file *m,
 
 	print_event_info(buf, m);
 
-	seq_printf(m, "#           TASK-PID   CPU#   %s  TIMESTAMP  FUNCTION\n", tgid ? "TGID     " : "");
-	seq_printf(m, "#              | |       |    %s     |         |\n",	 tgid ? "  |      " : "");
+	seq_printf(m, "#           TASK-PID   %s  CPU#   TIMESTAMP  FUNCTION\n", tgid ? "TGID     " : "");
+	seq_printf(m, "#              | |     %s    |       |         |\n",	 tgid ? "  |      " : "");
 }
 
 static void print_func_help_header_irq(struct trace_buffer *buf, struct seq_file *m,
@@ -3385,9 +3385,9 @@ static void print_func_help_header_irq(struct trace_buffer *buf, struct seq_file
 		   tgid ? tgid_space : space);
 	seq_printf(m, "#                          %s||| /     delay\n",
 		   tgid ? tgid_space : space);
-	seq_printf(m, "#           TASK-PID   CPU#%s||||    TIMESTAMP  FUNCTION\n",
+	seq_printf(m, "#           TASK-PID %sCPU#  ||||    TIMESTAMP  FUNCTION\n",
 		   tgid ? "   TGID   " : space);
-	seq_printf(m, "#              | |       | %s||||       |         |\n",
+	seq_printf(m, "#              | |   %s  |   ||||       |         |\n",
 		   tgid ? "     |    " : space);
 }
 
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 90db994ac900..1c8e30fda46a 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -594,8 +594,7 @@ int trace_print_context(struct trace_iterator *iter)
 
 	trace_find_cmdline(entry->pid, comm);
 
-	trace_seq_printf(s, "%16s-%-5d [%03d] ",
-			       comm, entry->pid, iter->cpu);
+	trace_seq_printf(s, "%16s-%-5d ", comm, entry->pid);
 
 	if (tr->trace_flags & TRACE_ITER_RECORD_TGID) {
 		unsigned int tgid = trace_find_tgid(entry->pid);
@@ -606,6 +605,8 @@ int trace_print_context(struct trace_iterator *iter)
 			trace_seq_printf(s, "(%5d) ", tgid);
 	}
 
+	trace_seq_printf(s, "[%03d] ", iter->cpu);
+
 	if (tr->trace_flags & TRACE_ITER_IRQ_INFO)
 		trace_print_lat_fmt(s, entry);
 
-- 
2.18.0.rc2.346.g013aa6912e-goog