linux-kernel - Re: [PATCH tip/core/rcu 04/23] rcu: Allow RCU grace-period cleanup to be preempted

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Message-ID: <20120902013631.GD5713@leaf>
Date:	Sat, 1 Sep 2012 18:36:31 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org
Subject: Re: [PATCH tip/core/rcu 04/23] rcu: Allow RCU grace-period cleanup
 to be preempted

On Thu, Aug 30, 2012 at 11:18:19AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> RCU grace-period cleanup is currently carried out with interrupts
> disabled, which can result in excessive latency spikes on large systems
> (many hundreds or thousands of CPUs).  This patch therefore makes the
> RCU grace-period cleanup be preemptible, including voluntary preemption
> points, which should eliminate those latency spikes.  Similar spikes from
> forcing of quiescent states will be dealt with similarly by later patches.
> 
> Reported-by: Mike Galbraith <mgalbraith@...e.de>
> Reported-by: Dimitri Sivanich <sivanich@....com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  kernel/rcutree.c |   11 +++++------
>  1 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 9fad21c..300aba6 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1170,7 +1170,7 @@ static int rcu_gp_kthread(void *arg)
>  		 * completed.
>  		 */
>  		if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
> -			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +			raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  
>  			/*
>  			 * Propagate new ->completed value to rcu_node
> @@ -1179,14 +1179,13 @@ static int rcu_gp_kthread(void *arg)
>  			 * to process their callbacks.
>  			 */
>  			rcu_for_each_node_breadth_first(rsp, rnp) {
> -				/* irqs already disabled. */
> -				raw_spin_lock(&rnp->lock);
> +				raw_spin_lock_irqsave(&rnp->lock, flags);
>  				rnp->completed = rsp->gpnum;
> -				/* irqs remain disabled. */
> -				raw_spin_unlock(&rnp->lock);
> +				raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +				cond_resched();
>  			}
>  			rnp = rcu_get_root(rsp);
> -			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> +			raw_spin_lock_irqsave(&rnp->lock, flags);
>  		}
>  
>  		rsp->completed = rsp->gpnum; /* Declare grace period done. */
> -- 
> 1.7.8
> 
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