linux-kernel - Re: [PATCH tip/core/rcu 2/2] rcu: Don't offload callbacks unless specifically requested

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Date:	Fri, 11 Jul 2014 15:47:37 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, oleg@...hat.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/rcu 2/2] rcu: Don't offload callbacks unless
 specifically requested

On Fri, Jul 11, 2014 at 06:35:03AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> Enabling NO_HZ_FULL currently has the side effect of enabling callback
> offloading on all CPUs.  This results in lots of additional rcuo kthreads,
> and can also increase context switching and wakeups, even in cases where
> callback offloading is neither needed nor particularly desirable.  This
> commit therefore enables callback offloading on a given CPU only if
> specifically requested at build time or boot time, or if that CPU has
> been specifically designated (again, either at build time or boot time)
> as a nohz_full CPU.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

ACK.

> ---
>  init/Kconfig             | 4 ++--
>  kernel/rcu/tree_plugin.h | 3 +++
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index 9d76b99af1b9..9332d33346ac 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -737,7 +737,7 @@ choice
>  
>  config RCU_NOCB_CPU_NONE
>  	bool "No build_forced no-CBs CPUs"
> -	depends on RCU_NOCB_CPU && !NO_HZ_FULL
> +	depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
>  	help
>  	  This option does not force any of the CPUs to be no-CBs CPUs.
>  	  Only CPUs designated by the rcu_nocbs= boot parameter will be
> @@ -751,7 +751,7 @@ config RCU_NOCB_CPU_NONE
>  
>  config RCU_NOCB_CPU_ZERO
>  	bool "CPU 0 is a build_forced no-CBs CPU"
> -	depends on RCU_NOCB_CPU && !NO_HZ_FULL
> +	depends on RCU_NOCB_CPU && !NO_HZ_FULL_ALL
>  	help
>  	  This option forces CPU 0 to be a no-CBs CPU, so that its RCU
>  	  callbacks are invoked by a per-CPU kthread whose name begins
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index b27b86c7bbfa..17eed0856b03 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2473,6 +2473,9 @@ static void __init rcu_spawn_nocb_kthreads(struct rcu_state *rsp)
>  
>  	if (rcu_nocb_mask == NULL)
>  		return;
> +#ifdef CONFIG_NO_HZ_FULL
> +	cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> +#endif /* #ifdef CONFIG_NO_HZ_FULL */
>  	if (ls == -1) {
>  		ls = int_sqrt(nr_cpu_ids);
>  		rcu_nocb_leader_stride = ls;
> -- 
> 1.8.1.5
> 
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