rcu: Enable RCU tracepoints by default to aid in debugging

While debugging a performance issue I needed to understand why
RCU sofitrqs were firing so frequently.

Unfortunately, the RCU callback tracepoints are hidden behind
CONFIG_RCU_TRACE which defaults to off in the upstream kernel and is
likely to also be disabled in enterprise distribution configs.

Enable it by default for CONFIG_TREE_RCU. However, we must keep it
disabled for tiny RCU, because it would otherwise pull in a large
amount of code that would make tiny RCU less than tiny.

I ran some file system metadata intensive workloads (git checkout,
FS-Mark) on a variety of machines with this patch and saw no
detectable change in performance.

Cc: Mel Gorman <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index b06848a104e6940e128e63a5d3c8c7fb39ffaf8b..5655bd95919abc2810f5ac158fa290a261e81f9b 100644 (file)
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1450,6 +1450,7 @@ config RCU_CPU_STALL_TIMEOUT
 config RCU_TRACE
        bool "Enable tracing for RCU"
        depends on DEBUG_KERNEL
+       default y if TREE_RCU
        select TRACE_CLOCK
        help
          This option provides tracing in RCU which presents stats