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Commit 755609a9 authored by Paul E. McKenney's avatar Paul E. McKenney
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rcu: Allow RCU grace-period initialization to be preempted 



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: default avatarMike Galbraith <mgalbraith@suse.de>
Reported-by: default avatarDimitri Sivanich <sivanich@sgi.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent 79bce672

kernel/rcutree.c

+11 −15
Original line number Diff line number Diff line
@@ -1030,7 +1030,7 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat 
/*

 * Body of kthread that handles grace periods.

 */

static int rcu_gp_kthread(void *arg)

static int __noreturn rcu_gp_kthread(void *arg)

{

	struct rcu_data *rdp;

	struct rcu_node *rnp;
@@ -1056,6 +1056,7 @@ static int rcu_gp_kthread(void *arg) 
			 * don't start another one.

			 */

			raw_spin_unlock_irq(&rnp->lock);

			cond_resched();

			continue;

		}
@@ -1066,6 +1067,7 @@ static int rcu_gp_kthread(void *arg) 
			 */

			rsp->fqs_need_gp = 1;

			raw_spin_unlock_irq(&rnp->lock);

			cond_resched();

			continue;

		}
@@ -1076,10 +1078,10 @@ 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_irq(&rnp->lock);



		/* 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
@@ -1091,15 +1093,9 @@ static int rcu_gp_kthread(void *arg) 
		 * indicate that no grace period is in progress, at least

		 * until the corresponding leaf node has been initialized.

		 * In addition, we have excluded CPU-hotplug operations.

		 *

		 * Note that the grace period cannot complete until

		 * we finish the initialization process, as there will

		 * be at least one qsmask bit set in the root node until

		 * that time, namely the one corresponding to this CPU,

		 * 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_irq(&rnp->lock);

			rcu_preempt_check_blocked_tasks(rnp);

			rnp->qsmask = rnp->qsmaskinit;

			rnp->gpnum = rsp->gpnum;
@@ -1110,17 +1106,17 @@ 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_irq(&rnp->lock);

			cond_resched();

		}



		rnp = rcu_get_root(rsp);

		raw_spin_lock(&rnp->lock); /* irqs already disabled. */

		raw_spin_lock_irq(&rnp->lock);

		/* force_quiescent_state() now OK. */

		rsp->fqs_state = RCU_SIGNAL_INIT;

		raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */

		raw_spin_unlock_irq(&rsp->onofflock);

		raw_spin_unlock_irq(&rnp->lock);

		put_online_cpus();

	}

	return 0;

}



/*