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Subject[PATCH tip/core/rcu 03/23] 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>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
---
kernel/rcutree.c | 26 +++++++++++---------------
1 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 0df9aaa..59c528f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1028,7 +1028,7 @@ 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;
@@ -1054,6 +1054,7 @@ static int rcu_gp_kthread(void *arg)
* don't start another one.
*/
raw_spin_unlock_irq(&rnp->lock);
+ cond_resched();
continue;
}

@@ -1064,6 +1065,7 @@ static int rcu_gp_kthread(void *arg)
*/
rsp->fqs_need_gp = 1;
raw_spin_unlock_irq(&rnp->lock);
+ cond_resched();
continue;
}

@@ -1074,10 +1076,10 @@ 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
@@ -1089,15 +1091,9 @@ 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;
@@ -1108,17 +1104,17 @@ 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;
}

/*
--
1.7.8


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