ftrace: display start of CPU buffer in trace output

Impact: change in trace output

Because the trace buffers are per cpu ring buffers, the start of
the trace can be confusing. If one CPU is very active at the
end of the trace, its history will not go as far back as the
other CPU traces.  This means that output for a particular CPU
may not appear for the first part of a trace.

To help annotate what is happening, and to prevent any more
confusion, this patch adds a line that annotates the start of
a CPU buffer output.

For example:

       automount-3495  [001]   184.596443: dnotify_parent <-vfs_write
[...]
       automount-3495  [001]   184.596449: dput <-path_put
       automount-3496  [002]   184.596450: down_read_trylock <-do_page_fault
[...]
           sshd-3497  [001]   184.597069: up_read <-do_page_fault
          <idle>-0     [000]   184.597074: __exit_idle <-exit_idle
[...]
       automount-3496  [002]   184.597257: filemap_fault <-__do_fault
          <idle>-0     [003]   184.597261: exit_idle <-smp_apic_timer_interrupt

Note, parsers of a trace output should always ignore any lines that
start with a '#'.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index cfda9d219e66..978145088fb8 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -277,6 +277,8 @@ struct trace_iterator {
         unsigned long           iter_flags;
         loff_t                  pos;
         long                    idx;
+
+        cpumask_t               started;
 };
 
 int tracing_is_enabled(void);