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

Message ID 1346350718-30937-2-git-send-email-paulmck@linux.vnet.ibm.com
State New
Headers show

Commit Message

Paul E. McKenney Aug. 30, 2012, 6:18 p.m.
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>
---
 kernel/rcutree.c |   17 ++++++++++-------
 1 files changed, 10 insertions(+), 7 deletions(-)

Comments

Josh Triplett Sept. 2, 2012, 1:09 a.m. | #1
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
>
Paul E. McKenney Sept. 5, 2012, 1:22 a.m. | #2
On Sat, Sep 01, 2012 at 06:09:35PM -0700, Josh Triplett wrote:
> 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?

Given that the event might have already signaled, I need to put the
cond_resched() into the loop.  Otherwise, we get back long latencies.

							Thanx, Paul

> 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
> > 
>

Patch

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;
 }