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Message-Id: <1348166900-18716-13-git-send-email-paulmck@linux.vnet.ibm.com>
Date: Thu, 20 Sep 2012 11:48:09 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
fweisbec@...il.com, sbw@....edu, patches@...aro.org,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 13/23] rcu: Prevent force_quiescent_state() memory contention
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Large systems running RCU_FAST_NO_HZ kernels see extreme memory
contention on the rcu_state structure's ->fqslock field. This
can be avoided by disabling RCU_FAST_NO_HZ, either at compile time
or at boot time (via the nohz kernel boot parameter), but large
systems will no doubt become sensitive to energy consumption.
This commit therefore uses a combining-tree approach to spread the
memory contention across new cache lines in the leaf rcu_node structures.
This can be thought of as a tournament lock that has only a try-lock
acquisition primitive.
The effect on small systems is minimal, because such systems have
an rcu_node "tree" consisting of a single node. In addition, this
functionality is not used on fastpaths.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@...htriplett.org>
---
kernel/rcutree.c | 47 +++++++++++++++++++++++++++++++++++++----------
kernel/rcutree.h | 1 +
2 files changed, 38 insertions(+), 10 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index b353d32..5edbdf8 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -61,6 +61,7 @@
/* Data structures. */
static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
+static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
#define RCU_STATE_INITIALIZER(sname, cr) { \
.level = { &sname##_state.node[0] }, \
@@ -1805,16 +1806,35 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
static void force_quiescent_state(struct rcu_state *rsp)
{
unsigned long flags;
- struct rcu_node *rnp = rcu_get_root(rsp);
+ bool ret;
+ struct rcu_node *rnp;
+ struct rcu_node *rnp_old = NULL;
+
+ /* Funnel through hierarchy to reduce memory contention. */
+ rnp = per_cpu_ptr(rsp->rda, raw_smp_processor_id())->mynode;
+ for (; rnp != NULL; rnp = rnp->parent) {
+ ret = (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) ||
+ !raw_spin_trylock(&rnp->fqslock);
+ if (rnp_old != NULL)
+ raw_spin_unlock(&rnp_old->fqslock);
+ if (ret) {
+ rsp->n_force_qs_lh++;
+ return;
+ }
+ rnp_old = rnp;
+ }
+ /* rnp_old == rcu_get_root(rsp), rnp == NULL. */
- if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS)
+ /* Reached the root of the rcu_node tree, acquire lock. */
+ raw_spin_lock_irqsave(&rnp_old->lock, flags);
+ raw_spin_unlock(&rnp_old->fqslock);
+ if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
+ rsp->n_force_qs_lh++;
+ raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
return; /* Someone beat us to it. */
- if (!raw_spin_trylock_irqsave(&rnp->lock, flags)) {
- rsp->n_force_qs_lh++; /* Inexact, can lose counts. Tough! */
- return;
}
rsp->gp_flags |= RCU_GP_FLAG_FQS;
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
wake_up(&rsp->gp_wq); /* Memory barrier implied by wake_up() path. */
}
@@ -2702,10 +2722,14 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
static void __init rcu_init_one(struct rcu_state *rsp,
struct rcu_data __percpu *rda)
{
- static char *buf[] = { "rcu_node_level_0",
- "rcu_node_level_1",
- "rcu_node_level_2",
- "rcu_node_level_3" }; /* Match MAX_RCU_LVLS */
+ static char *buf[] = { "rcu_node_0",
+ "rcu_node_1",
+ "rcu_node_2",
+ "rcu_node_3" }; /* Match MAX_RCU_LVLS */
+ static char *fqs[] = { "rcu_node_fqs_0",
+ "rcu_node_fqs_1",
+ "rcu_node_fqs_2",
+ "rcu_node_fqs_3" }; /* Match MAX_RCU_LVLS */
int cpustride = 1;
int i;
int j;
@@ -2730,6 +2754,9 @@ static void __init rcu_init_one(struct rcu_state *rsp,
raw_spin_lock_init(&rnp->lock);
lockdep_set_class_and_name(&rnp->lock,
&rcu_node_class[i], buf[i]);
+ raw_spin_lock_init(&rnp->fqslock);
+ lockdep_set_class_and_name(&rnp->fqslock,
+ &rcu_fqs_class[i], fqs[i]);
rnp->gpnum = 0;
rnp->qsmask = 0;
rnp->qsmaskinit = 0;
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 7fb93ce..8f0293c 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -202,6 +202,7 @@ struct rcu_node {
/* per-CPU kthreads as needed. */
unsigned int node_kthread_status;
/* State of node_kthread_task for tracing. */
+ raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
} ____cacheline_internodealigned_in_smp;
/*
--
1.7.8
--
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