linux-kernel - Re: [PATCH 14/16] rcu: Remove redundant checks for rcu_scheduler_fully

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Message-ID: <20140723122752.GM11241@linux.vnet.ibm.com>
Date:	Wed, 23 Jul 2014 05:27:52 -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 14/16] rcu: Remove redundant checks for
 rcu_scheduler_fully_active

On Wed, Jul 23, 2014 at 01:09:51AM -0400, Pranith Kumar wrote:
> rcu_scheduler_fully_active is set to true early in the boot process.
> 
> The check for this flag is only needed in rcu_prepare_kthreads() as this
> function is called before the above flag is set. All other checks are redundant.
> Hence this commit removes those redundant checks.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@...il.com>

This might actually work, but the checks are cheap, don't happen very often,
and make it much easier to modify the code.  So sorry, but no.

							Thanx, Paul

> ---
>  kernel/rcu/tree_plugin.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 9a50e1d..541c776 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -1341,7 +1341,7 @@ static void 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;
> @@ -1486,9 +1486,10 @@ 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)
> -		rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
> +	if (!rcu_scheduler_fully_active)
> +		return;
> 
> +	rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
>  	rcu_spawn_all_nocb_kthreads(cpu);
>  }
> 
> @@ -2508,9 +2509,8 @@ 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);
>  }
> 
>  /*
> -- 
> 2.0.0.rc2
> 

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