[PATCH 5/5] ftrace: Update dynamic ftrace calls only if necessary

[Jiri Olsa]

Currently dynamic ftrace calls are updated any time
the ftrace_ops is un/registered. If we do  this update
only when it's needed, we save lot of time for perf
system wide ftrace function sampling/counting.

The reason is that for system wide sampling/counting,
perf creates event for each cpu in the system.

Each event then registers separate copy of ftrace_ops,
which ends up in FTRACE_UPDATE_CALLS updates. On servers
with many cpus that means serious stall (240 cpus server):

Counting:
  # time ./perf stat -e ftrace:function -a sleep 1

   Performance counter stats for 'system wide':

              370,663      ftrace:function

          1.401427505 seconds time elapsed

  real    3m51.743s
  user    0m0.023s
  sys     3m48.569s

Sampling:
  # time ./perf record -e ftrace:function -a sleep 1
  [ perf record: Woken up 0 times to write data ]
  Warning:
  Processed 141200 events and lost 5 chunks!

  [ perf record: Captured and wrote 10.703 MB perf.data (135950 samples) ]

  real    2m31.429s
  user    0m0.213s
  sys     2m29.494s

There's no reason to do the FTRACE_UPDATE_CALLS update
for each event in perf case, because all the ftrace_ops
always share the same filter, so the updated calls are
always the same.

It's required that only first ftrace_ops registration
does the FTRACE_UPDATE_CALLS update (also sometimes
the second if the first one used the trampoline), but
the rest can be only cheaply linked into the ftrace_ops
list.

Counting:
  # time ./perf stat -e ftrace:function -a sleep 1

   Performance counter stats for 'system wide':

             398,571      ftrace:function

         1.377503733 seconds time elapsed

  real    0m2.787s
  user    0m0.005s
  sys     0m1.883s

Sampling:
  # time ./perf record -e ftrace:function -a sleep 1
  [ perf record: Woken up 0 times to write data ]
  Warning:
  Processed 261730 events and lost 9 chunks!

  [ perf record: Captured and wrote 19.907 MB perf.data (256293 samples) ]

  real    1m31.948s
  user    0m0.309s
  sys     1m32.051s

Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
 kernel/trace/ftrace.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 123dddc660e9..48b491463549 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -2650,7 +2650,6 @@ static int ftrace_startup(struct ftrace_ops *ops, int command)
 		return ret;
 
 	ftrace_start_up++;
-	command |= FTRACE_UPDATE_CALLS;
 
 	/*
 	 * Note that ftrace probes uses this to start up
@@ -2671,7 +2670,8 @@ static int ftrace_startup(struct ftrace_ops *ops, int command)
 		return ret;
 	}
 
-	ftrace_hash_rec_enable(ops, 1);
+	if (ftrace_hash_rec_enable(ops, 1))
+		command |= FTRACE_UPDATE_CALLS;
 
 	ftrace_startup_enable(command);
 
@@ -2701,11 +2701,11 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
 
 	/* Disabling ipmodify never fails */
 	ftrace_hash_ipmodify_disable(ops);
-	ftrace_hash_rec_disable(ops, 1);
 
-	ops->flags &= ~FTRACE_OPS_FL_ENABLED;
+	if (ftrace_hash_rec_disable(ops, 1))
+		command |= FTRACE_UPDATE_CALLS;
 
-	command |= FTRACE_UPDATE_CALLS;
+	ops->flags &= ~FTRACE_OPS_FL_ENABLED;
 
 	if (saved_ftrace_func != ftrace_trace_function) {
 		saved_ftrace_func = ftrace_trace_function;
-- 
2.4.3