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Message-ID: <20120314002414.GA21561@linux.vnet.ibm.com>
Date: Tue, 13 Mar 2012 17:24:14 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: sivanich@....com
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH RFC] rcu: Limit GP initialization to CPUs that have been
online
The following builds, but is only very lightly tested. Probably full
of bug, especially when exercising CPU hotplug.
Thanx, Paul
------------------------------------------------------------------------
The current grace-period initialization initializes all leaf rcu_node
structures, even those corresponding to CPUs that have never been online.
This is harmless in many configurations, but results in 200-microsecond
latency spikes for kernels built with NR_CPUS=4096.
This commit therefore keeps track of the largest-numbered CPU that has
ever been online, and limits grace-period initialization to rcu_node
structures corresponding to that CPU and to smaller-numbered CPUs.
Reported-by: Dimitri Sivanich <sivanich@....com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index c3b05ef..5688443 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -91,6 +91,8 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
static struct rcu_state *rcu_state;
+int rcu_max_cpu __read_mostly; /* Largest # CPU that has ever been online. */
+
/*
* The rcu_scheduler_active variable transitions from zero to one just
* before the first task is spawned. So when this variable is zero, RCU
@@ -1129,8 +1131,9 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
__releases(rcu_get_root(rsp)->lock)
{
unsigned long gp_duration;
- struct rcu_node *rnp = rcu_get_root(rsp);
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
+ struct rcu_node *rnp;
+ struct rcu_node *rnp_root = rcu_get_root(rsp);
WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
@@ -1159,26 +1162,28 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
* completed.
*/
if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
- raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
/*
* Propagate new ->completed value to rcu_node structures
* so that other CPUs don't have to wait until the start
* of the next grace period to process their callbacks.
+ * We must hold the root rcu_node structure's ->lock
+ * across rcu_for_each_node_breadth_first() in order to
+ * synchronize with CPUs coming online for the first time.
*/
rcu_for_each_node_breadth_first(rsp, rnp) {
+ raw_spin_unlock(&rnp_root->lock); /* remain disabled. */
raw_spin_lock(&rnp->lock); /* irqs already disabled. */
rnp->completed = rsp->gpnum;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ raw_spin_lock(&rnp_root->lock); /* already disabled. */
}
- rnp = rcu_get_root(rsp);
- raw_spin_lock(&rnp->lock); /* irqs already disabled. */
}
rsp->completed = rsp->gpnum; /* Declare the grace period complete. */
trace_rcu_grace_period(rsp->name, rsp->completed, "end");
rsp->fqs_state = RCU_GP_IDLE;
- rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
+ rcu_start_gp(rsp, flags); /* releases root node's ->lock. */
}
/*
@@ -2440,6 +2445,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
unsigned long mask;
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_node *rnp = rcu_get_root(rsp);
+ struct rcu_node *rnp_init;
/* Set up local state, ensuring consistent view of global state. */
raw_spin_lock_irqsave(&rnp->lock, flags);
@@ -2462,6 +2468,16 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
/* Exclude any attempts to start a new GP on large systems. */
raw_spin_lock(&rsp->onofflock); /* irqs already disabled. */
+ /* Initialize any rcu_node structures that will see their first use. */
+ raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+ for (rnp_init = per_cpu_ptr(rsp->rda, rcu_max_cpu)->mynode + 1;
+ rnp_init <= rdp->mynode;
+ rnp_init++) {
+ rnp_init->gpnum = rsp->gpnum;
+ rnp_init->completed = rsp->completed;
+ }
+ raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+
/* Add CPU to rcu_node bitmasks. */
rnp = rdp->mynode;
mask = rdp->grpmask;
@@ -2495,6 +2511,8 @@ static void __cpuinit rcu_prepare_cpu(int cpu)
rcu_init_percpu_data(cpu, &rcu_sched_state, 0);
rcu_init_percpu_data(cpu, &rcu_bh_state, 0);
rcu_preempt_init_percpu_data(cpu);
+ if (cpu > rcu_max_cpu)
+ rcu_max_cpu = cpu;
}
/*
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 1e49c56..1dc74e0 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -192,11 +192,13 @@ struct rcu_node {
/*
* Do a full breadth-first scan of the rcu_node structures for the
- * specified rcu_state structure.
+ * specified rcu_state structure. The caller must hold either the
+ * ->onofflock or the root rcu_node structure's ->lock.
*/
+extern int rcu_max_cpu;
#define rcu_for_each_node_breadth_first(rsp, rnp) \
for ((rnp) = &(rsp)->node[0]; \
- (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+ (rnp) < per_cpu_ptr((rsp)->rda, rcu_max_cpu)->mynode; (rnp)++)
/*
* Do a breadth-first scan of the non-leaf rcu_node structures for the
--
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