linux-kernel - [PATCH] rcu: Enable RCU tracepoints by default to aid in debugging

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Message-Id: <20161223151753.22745-1-matt@codeblueprint.co.uk>
Date:   Fri, 23 Dec 2016 15:17:53 +0000
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH] 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 <mgorman@...hsingularity.net>
Signed-off-by: Matt Fleming <matt@...eblueprint.co.uk>
---
 lib/Kconfig.debug | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a6c8db1d62f6..7fe1ff001792 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1431,6 +1431,7 @@ 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
-- 
2.10.0