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, 28 insertions, 2 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index e8e6043f4d29..09b4fa6e4d3b 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -170,6 +170,26 @@ int ftrace_event_reg(struct ftrace_event_call *call, enum trace_reg type)
 }
 EXPORT_SYMBOL_GPL(ftrace_event_reg);
 
+void trace_event_enable_cmd_record(bool enable)
+{
+        struct ftrace_event_call *call;
+
+        mutex_lock(&event_mutex);
+        list_for_each_entry(call, &ftrace_events, list) {
+                if (!(call->flags & TRACE_EVENT_FL_ENABLED))
+                        continue;
+
+                if (enable) {
+                        tracing_start_cmdline_record();
+                        call->flags |= TRACE_EVENT_FL_RECORDED_CMD;
+                } else {
+                        tracing_stop_cmdline_record();
+                        call->flags &= ~TRACE_EVENT_FL_RECORDED_CMD;
+                }
+        }
+        mutex_unlock(&event_mutex);
+}
+
 static int ftrace_event_enable_disable(struct ftrace_event_call *call,
                                         int enable)
 {
@@ -179,13 +199,19 @@ static int ftrace_event_enable_disable(struct ftrace_event_call *call,
         case 0:
                 if (call->flags & TRACE_EVENT_FL_ENABLED) {
                         call->flags &= ~TRACE_EVENT_FL_ENABLED;
-                        tracing_stop_cmdline_record();
+                        if (call->flags & TRACE_EVENT_FL_RECORDED_CMD) {
+                                tracing_stop_cmdline_record();
+                                call->flags &= ~TRACE_EVENT_FL_RECORDED_CMD;
+                        }
                         call->class->reg(call, TRACE_REG_UNREGISTER);
                 }
                 break;
         case 1:
                 if (!(call->flags & TRACE_EVENT_FL_ENABLED)) {
-                        tracing_start_cmdline_record();
+                        if (trace_flags & TRACE_ITER_RECORD_CMD) {
+                                tracing_start_cmdline_record();
+                                call->flags |= TRACE_EVENT_FL_RECORDED_CMD;
+                        }
                         ret = call->class->reg(call, TRACE_REG_REGISTER);
                         if (ret) {
                                 tracing_stop_cmdline_record();