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rcu: Reduce synchronize_rcu_expedited() latency 

The synchronize_rcu_expedited() function disables interrupts across a
scan of all leaf rcu_node structures, which is not good for real-time
scheduling latency on large systems (hundreds or especially thousands
of CPUs).  This commit therefore holds off CPU-hotplug operations using
get_online_cpus(), and removes the prior acquisiion of the ->onofflock
(which required disabling interrupts).

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 2012-07-30 17:19:25 -07:00
parent bcfa57ce10
commit 1943c89de7
1 changed files with 22 additions and 8 deletions
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30
kernel/rcutree_plugin.h
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@ -799,34 +799,48 @@ void synchronize_rcu_expedited(void)
snap = ACCESS_ONCE(sync_rcu_preempt_exp_count) + 1;
smp_mb(); /* Above access cannot bleed into critical section. */
/*
* Block CPU-hotplug operations. This means that any CPU-hotplug
* operation that finds an rcu_node structure with tasks in the
* process of being boosted will know that all tasks blocking
* this expedited grace period will already be in the process of
* being boosted. This simplifies the process of moving tasks
* from leaf to root rcu_node structures.
*/
get_online_cpus();
/*
* Acquire lock, falling back to synchronize_rcu() if too many
* lock-acquisition failures. Of course, if someone does the
* expedited grace period for us, just leave.
*/
while (!mutex_trylock(&sync_rcu_preempt_exp_mutex)) {
if (ULONG_CMP_LT(snap,
ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
put_online_cpus();
goto mb_ret; /* Others did our work for us. */
}
if (trycount++ < 10) {
udelay(trycount * num_online_cpus());
} else {
put_online_cpus();
synchronize_rcu();
return;
}
if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
goto mb_ret; /* Others did our work for us. */
}
if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count)))
if (ULONG_CMP_LT(snap, ACCESS_ONCE(sync_rcu_preempt_exp_count))) {
put_online_cpus();
goto unlock_mb_ret; /* Others did our work for us. */
}
/* force all RCU readers onto ->blkd_tasks lists. */
synchronize_sched_expedited();
raw_spin_lock_irqsave(&rsp->onofflock, flags);
/* Initialize ->expmask for all non-leaf rcu_node structures. */
rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) {
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
raw_spin_lock_irqsave(&rnp->lock, flags);
rnp->expmask = rnp->qsmaskinit;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
raw_spin_unlock_irqrestore(&rnp->lock, flags);
}
/* Snapshot current state of ->blkd_tasks lists. */
@ -835,7 +849,7 @@ void synchronize_rcu_expedited(void)
if (NUM_RCU_NODES > 1)
sync_rcu_preempt_exp_init(rsp, rcu_get_root(rsp));
raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
put_online_cpus();
/* Wait for snapshotted ->blkd_tasks lists to drain. */
rnp = rcu_get_root(rsp);