linux-kernel - Re: [PATCH tip/core/rcu 10/23] rcu: Allow RCU quiescent-state forcing to be preempted

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Date:	Sat, 1 Sep 2012 22:23:02 -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 10/23] rcu: Allow RCU quiescent-state
 forcing to be preempted

On Thu, Aug 30, 2012 at 11:18:25AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> 
> RCU quiescent-state forcing is currently carried out without preemption
> points, which can result in excessive latency spikes on large systems
> (many hundreds or thousands of CPUs).  This patch therefore inserts
> a voluntary preemption point into force_qs_rnp(), which should greatly
> reduce the magnitude of these spikes.
> 
> 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 |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 79c2c28..cce73ff 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1784,6 +1784,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
>  	struct rcu_node *rnp;
>  
>  	rcu_for_each_leaf_node(rsp, rnp) {
> +		cond_resched();
>  		mask = 0;
>  		raw_spin_lock_irqsave(&rnp->lock, flags);
>  		if (!rcu_gp_in_progress(rsp)) {
> -- 
> 1.7.8
> 
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