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rcu: Allow RCU grace-period initialization to be preempted

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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>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2012-06-19 17:18:20 -07:00
parent 79bce67243
commit 755609a908
1 changed files with 11 additions and 15 deletions
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26
kernel/rcutree.c
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@ -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. * 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_data *rdp;
struct rcu_node *rnp; struct rcu_node *rnp;
@ -1056,6 +1056,7 @@ static int rcu_gp_kthread(void *arg)
* don't start another one. * don't start another one.
*/ */
raw_spin_unlock_irq(&rnp->lock); raw_spin_unlock_irq(&rnp->lock);
cond_resched();
continue; continue;
} }
@ -1066,6 +1067,7 @@ static int rcu_gp_kthread(void *arg)
*/ */
rsp->fqs_need_gp = 1; rsp->fqs_need_gp = 1;
raw_spin_unlock_irq(&rnp->lock); raw_spin_unlock_irq(&rnp->lock);
cond_resched();
continue; continue;
} }
@ -1076,10 +1078,10 @@ static int rcu_gp_kthread(void *arg)
rsp->fqs_state = RCU_GP_INIT; /* Stop force_quiescent_state. */ rsp->fqs_state = RCU_GP_INIT; /* Stop force_quiescent_state. */
rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS; rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
record_gp_stall_check_time(rsp); 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. */ /* 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 * 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 * indicate that no grace period is in progress, at least
* until the corresponding leaf node has been initialized. * until the corresponding leaf node has been initialized.
* In addition, we have excluded CPU-hotplug operations. * 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) { 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); rcu_preempt_check_blocked_tasks(rnp);
rnp->qsmask = rnp->qsmaskinit; rnp->qsmask = rnp->qsmaskinit;
rnp->gpnum = rsp->gpnum; rnp->gpnum = rsp->gpnum;
@ -1110,17 +1106,17 @@ static int rcu_gp_kthread(void *arg)
trace_rcu_grace_period_init(rsp->name, rnp->gpnum, trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
rnp->level, rnp->grplo, rnp->level, rnp->grplo,
rnp->grphi, rnp->qsmask); 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); rnp = rcu_get_root(rsp);
raw_spin_lock(&rnp->lock); /* irqs already disabled. */ raw_spin_lock_irq(&rnp->lock);
/* force_quiescent_state() now OK. */ /* force_quiescent_state() now OK. */
rsp->fqs_state = RCU_SIGNAL_INIT; rsp->fqs_state = RCU_SIGNAL_INIT;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */ raw_spin_unlock_irq(&rnp->lock);
raw_spin_unlock_irq(&rsp->onofflock); put_online_cpus();
} }
return 0;
} }
/* /*