linux-kernel - Re: [PATCH 1/1] rcu: Remove redundant checks for rcu_scheduler_fully

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Message-ID: <20140717234027.GC8690@linux.vnet.ibm.com>
Date:	Thu, 17 Jul 2014 16:40:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"open list:READ-COPY UPDATE..." <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] rcu: Remove redundant checks for
 rcu_scheduler_fully_active

On Tue, Jul 15, 2014 at 09:50:02PM -0400, Pranith Kumar wrote:
> rcu_scheduler_fully_active is set to true early in the boot process.
> 
> rcu_prepare_kthreads() is called in two locations. Once after setting the above
> flag and the other while hotplugging a CPU from rcu_cpu_notify(). CPU hotplug is
> enabled much later by which time the above flag is already set. Hence checking
> for this flag is redundant in this function.
> 
> The checks in rcu_spawn_one_boost_kthread() and rcu_spawn_all_nocb_kthreads()
> are similarly redundant.
> 
> This commit removes the redundant checks for this flag from the above
> locations.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

Good catch!  Queued for 3.18.

							Thanx, Paul

> ---
>  kernel/rcu/tree_plugin.h | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 17ccb62..a2113f6 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1341,7 +1341,7 @@ static int rcu_spawn_one_boost_kthread(struct rcu_state *rsp,
>  	if (&rcu_preempt_state != rsp)
>  		return 0;
> 
> -	if (!rcu_scheduler_fully_active || rnp->qsmaskinit == 0)
> +	if (rnp->qsmaskinit == 0)
>  		return 0;
> 
>  	rsp->boost = 1;
> @@ -1489,8 +1489,7 @@ static void rcu_prepare_kthreads(int cpu)
>  	struct rcu_node *rnp = rdp->mynode;
> 
>  	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
> -	if (rcu_scheduler_fully_active)
> -		(void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
> +	(void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
>  }
> 
>  #else /* #ifdef CONFIG_RCU_BOOST */
> @@ -2509,9 +2508,8 @@ static void rcu_spawn_all_nocb_kthreads(int cpu)
>  {
>  	struct rcu_state *rsp;
> 
> -	if (rcu_scheduler_fully_active)
> -		for_each_rcu_flavor(rsp)
> -			rcu_spawn_one_nocb_kthread(rsp, cpu);
> +	for_each_rcu_flavor(rsp)
> +		rcu_spawn_one_nocb_kthread(rsp, cpu);
>  }
> 
>  /*
> -- 
> 1.9.1
> 

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