rcu: Reduce rcutorture tracing

Currently, rcutorture traces every read-side access.  This can be
problematic because even a two-minute rcutorture run on a two-CPU system
can generate 28,853,363 reads.  Normally, only a failing read is of
interest, so this commit traces adjusts rcutorture's tracing to only
trace failing reads.  The resulting event tracing records the time
and the ->completed value captured at the beginning of the RCU read-side
critical section, allowing correlation with other event-tracing messages.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
[ paulmck: Add fix to build problem located by Randy Dunlap based on
  diagnosis by Steven Rostedt. ]

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 3a353091a903..7d83f52fbade 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1008,6 +1008,7 @@ config RCU_CPU_STALL_INFO
 config RCU_TRACE
         bool "Enable tracing for RCU"
         depends on DEBUG_KERNEL
+        select TRACE_CLOCK
         help
           This option provides tracing in RCU which presents stats
           in debugfs for debugging RCU implementation.