tracing/lockdep: report the time waited for a lock

While trying to optimize the new lock on reiserfs to replace
the bkl, I find the lock tracing very useful though it lacks
something important for performance (and latency) instrumentation:
the time a task waits for a lock.

That's what this patch implements:

  bash-4816  [000]   202.652815: lock_contended: lock_contended: &sb->s_type->i_mutex_key
  bash-4816  [000]   202.652819: lock_acquired: &rq->lock (0.000 us)
 <...>-4787  [000]   202.652825: lock_acquired: &rq->lock (0.000 us)
 <...>-4787  [000]   202.652829: lock_acquired: &rq->lock (0.000 us)
  bash-4816  [000]   202.652833: lock_acquired: &sb->s_type->i_mutex_key (16.005 us)

As shown above, the "lock acquired" field is followed by the time
it has been waiting for the lock. Usually, a lock contended entry
is followed by a near lock_acquired entry with a non-zero time waited.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1238975373-15739-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 4 insertions, 4 deletions

diff --git a/kernel/lockdep.c b/kernel/lockdep.c
index b0f011866969..c4582a6ea953 100644
--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -3061,6 +3061,8 @@ found_it:
         put_lock_stats(stats);
 }
 
+DEFINE_TRACE(lock_acquired);
+
 static void
 __lock_acquired(struct lockdep_map *lock, unsigned long ip)
 {
@@ -3099,6 +3101,8 @@ found_it:
                 hlock->holdtime_stamp = now;
         }
 
+        trace_lock_acquired(lock, ip, waittime);
+
         stats = get_lock_stats(hlock_class(hlock));
         if (waittime) {
                 if (hlock->read)
@@ -3137,14 +3141,10 @@ void lock_contended(struct lockdep_map *lock, unsigned long ip)
 }
 EXPORT_SYMBOL_GPL(lock_contended);
 
-DEFINE_TRACE(lock_acquired);
-
 void lock_acquired(struct lockdep_map *lock, unsigned long ip)
 {
         unsigned long flags;
 
-        trace_lock_acquired(lock, ip);
-
         if (unlikely(!lock_stat))
                 return;