lkml.org 
[lkml]   [2012]   [Aug]   [30]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH tip/core/rcu 22/23] rcu: Reduce synchronize_rcu_expedited() latency
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

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>
---
kernel/rcutree_plugin.h | 30 ++++++++++++++++++++++--------
1 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 7ed45c9..f1e06f6 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -800,33 +800,47 @@ void synchronize_rcu_expedited(void)
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);
--
1.7.8


\
 
 \ /
  Last update: 2012-08-30 21:01    [W:0.352 / U:10.620 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site