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

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Message-Id: <20161223171625.GY3924@linux.vnet.ibm.com>
Date:   Fri, 23 Dec 2016 09:16:25 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Matt Fleming <matt@...eblueprint.co.uk>
Cc:     linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH] rcu: Enable RCU tracepoints by default to aid in
 debugging

On Fri, Dec 23, 2016 at 03:17:53PM +0000, Matt Fleming wrote:
> 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>

Queued for testing and review, thank you!

							Thanx, Paul

> ---
>  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
>