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Subject[PATCH tip/core/rcu 02/23] rcu: Allow RCU grace-period initialization to be preempted
Date
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(-)

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


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