Re: [PATCH tip/core/rcu 02/23] rcu: Allow RCU grace-period initialization to be preempted

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Aug 30, 2012 at 11:18:17AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> RCU grace-period initialization is currently carried out with interrupts
> disabled, which can result in 200-microsecond latency spikes on systems
> on which RCU has been configured for 4096 CPUs.  This patch therefore
> makes the RCU grace-period initialization be preemptible, which should
> eliminate those latency spikes.  Similar spikes from grace-period cleanup
> and the forcing of quiescent states will be dealt with similarly by later
> patches.
> 
> Reported-by: Mike Galbraith <mgalbraith@suse.de>
> Reported-by: Dimitri Sivanich <sivanich@sgi.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Does it make sense to have cond_resched() right before the continues,
which lead right back up to the wait_event_interruptible at the top of
the loop?  Or do you expect to usually find that event already
signalled?

In any case:

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

>  kernel/rcutree.c |   17 ++++++++++-------
>  1 files changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index e1c5868..ef56aa3 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1069,6 +1069,7 @@ static int rcu_gp_kthread(void *arg)
>  			 * don't start another one.
>  			 */
>  			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +			cond_resched();
>  			continue;
>  		}
>  
> @@ -1079,6 +1080,7 @@ static int rcu_gp_kthread(void *arg)
>  			 */
>  			rsp->fqs_need_gp = 1;
>  			raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +			cond_resched();
>  			continue;
>  		}
>  
> @@ -1089,10 +1091,10 @@ static int rcu_gp_kthread(void *arg)
>  		rsp->fqs_state = RCU_GP_INIT; /* Stop force_quiescent_state. */
>  		rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
>  		record_gp_stall_check_time(rsp);
> -		raw_spin_unlock(&rnp->lock);  /* leave irqs disabled. */
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
>  
>  		/* Exclude any concurrent CPU-hotplug operations. */
> -		raw_spin_lock(&rsp->onofflock);  /* irqs already disabled. */
> +		get_online_cpus();
>  
>  		/*
>  		 * Set the quiescent-state-needed bits in all the rcu_node
> @@ -1112,7 +1114,7 @@ static int rcu_gp_kthread(void *arg)
>  		 * due to the fact that we have irqs disabled.
>  		 */
>  		rcu_for_each_node_breadth_first(rsp, rnp) {
> -			raw_spin_lock(&rnp->lock); /* irqs already disabled. */
> +			raw_spin_lock_irqsave(&rnp->lock, flags);
>  			rcu_preempt_check_blocked_tasks(rnp);
>  			rnp->qsmask = rnp->qsmaskinit;
>  			rnp->gpnum = rsp->gpnum;
> @@ -1123,15 +1125,16 @@ static int rcu_gp_kthread(void *arg)
>  			trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
>  						    rnp->level, rnp->grplo,
>  						    rnp->grphi, rnp->qsmask);
> -			raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> +			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);
>  		/* force_quiescent_state() now OK. */
>  		rsp->fqs_state = RCU_SIGNAL_INIT;
> -		raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
> -		raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
> +		raw_spin_unlock_irqrestore(&rnp->lock, flags);
> +		put_online_cpus();
>  	}
>  	return 0;
>  }
> -- 
> 1.7.8
>