tracing: Allow to disable cmdline recording

We found that even enabling a single trace event that will rarely be
triggered can add big overhead to context switch.

(lmbench context switch test)
 -------------------------------------------------
 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
 ctxsw  ctxsw  ctxsw ctxsw  ctxsw   ctxsw   ctxsw
------ ------ ------ ------ ------ ------- -------
  2.19   2.3   2.21   2.56   2.13     2.54    2.07
  2.39   2.51  2.35   2.75   2.27     2.81    2.24

The overhead is 6% ~ 11%.

It's because when a trace event is enabled 3 tracepoints (sched_switch,
sched_wakeup, sched_wakeup_new) will be activated to map pid to cmdname.

We'd like to avoid this overhead, so add a trace option '(no)record-cmd'
to allow to disable cmdline recording.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4C2D57F4.2050204@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 01df7ca4ead7..2b7b1395b4d5 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -152,11 +152,13 @@ extern int ftrace_event_reg(struct ftrace_event_call *event,
 enum {
         TRACE_EVENT_FL_ENABLED_BIT,
         TRACE_EVENT_FL_FILTERED_BIT,
+        TRACE_EVENT_FL_RECORDED_CMD_BIT,
 };
 
 enum {
-        TRACE_EVENT_FL_ENABLED  = (1 << TRACE_EVENT_FL_ENABLED_BIT),
-        TRACE_EVENT_FL_FILTERED = (1 << TRACE_EVENT_FL_FILTERED_BIT),
+        TRACE_EVENT_FL_ENABLED          = (1 << TRACE_EVENT_FL_ENABLED_BIT),
+        TRACE_EVENT_FL_FILTERED         = (1 << TRACE_EVENT_FL_FILTERED_BIT),
+        TRACE_EVENT_FL_RECORDED_CMD     = (1 << TRACE_EVENT_FL_RECORDED_CMD_BIT),
 };
 
 struct ftrace_event_call {
@@ -174,6 +176,7 @@ struct ftrace_event_call {
          * 32 bit flags:
          *   bit 1:             enabled
          *   bit 2:             filter_active
+         *   bit 3:             enabled cmd record
          *
          * Changes to flags must hold the event_mutex.
          *