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Date: Fri, 16 Mar 2012 09:45:35 +0100
From: Mike Galbraith <efault@....de>
To: Dimitri Sivanich <sivanich@....com>
Cc: paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] rcu: Limit GP initialization to CPUs that have been
online
On Fri, 2012-03-16 at 09:09 +0100, Mike Galbraith wrote:
> On Fri, 2012-03-16 at 08:27 +0100, Mike Galbraith wrote:
> > On Thu, 2012-03-15 at 12:59 -0500, Dimitri Sivanich wrote:
> > > Could I try your 3.0 enterprise patch?
> >
> > Sure, v3 below.
>
> Erm, a bit of that went missing. I'll put it back.
OK, box finally finished rebuild/boot.
rcu: Limit GP initialization to CPUs that have been online
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>
Acked-by: Mike Galbraith <mgalbraith@...e.de>
---
kernel/rcutree.c | 36 ++++++++++++++++++++++++++++++++----
kernel/rcutree.h | 16 ++++++++++++++--
2 files changed, 46 insertions(+), 6 deletions(-)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -84,6 +84,8 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_d
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
@@ -827,25 +829,31 @@ rcu_start_gp(struct rcu_state *rsp, unsi
struct rcu_node *rnp = rcu_get_root(rsp);
if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
+ struct rcu_node *rnp_root = rnp;
+
if (cpu_needs_another_gp(rsp, rdp))
rsp->fqs_need_gp = 1;
if (rnp->completed == rsp->completed) {
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
+ raw_spin_unlock_irqrestore(&rnp_root->lock, flags);
return;
}
- 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->completed;
raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+ raw_spin_lock(&rnp_root->lock); /* already disabled. */
}
- local_irq_restore(flags);
+ raw_spin_unlock_irqrestore(&rnp_root->lock, flags);
return;
}
@@ -935,7 +943,7 @@ static void rcu_report_qs_rsp(struct rcu
rsp->gp_max = gp_duration;
rsp->completed = rsp->gpnum;
rsp->signaled = RCU_GP_IDLE;
- rcu_start_gp(rsp, flags); /* releases root node's rnp->lock. */
+ rcu_start_gp(rsp, flags); /* releases root node's ->lock. */
}
/*
@@ -1862,6 +1870,7 @@ rcu_init_percpu_data(int cpu, struct rcu
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);
@@ -1882,6 +1891,20 @@ rcu_init_percpu_data(int cpu, struct rcu
/* 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.
+ * Note that rcu_max_cpu cannot change out from under us because the
+ * hotplug locks are held.
+ */
+ 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;
@@ -1907,6 +1930,11 @@ static void __cpuinit rcu_prepare_cpu(in
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) {
+ smp_mb(); /* Initialization before rcu_max_cpu assignment. */
+ rcu_max_cpu = cpu;
+ smp_mb(); /* rcu_max_cpu assignment before later uses. */
+ }
}
/*
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -191,11 +191,23 @@ 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;
+static inline int rcu_get_max_cpu(void)
+{
+ int ret;
+
+ smp_mb(); /* Pairs with barriers in rcu_prepare_cpu(). */
+ ret = rcu_max_cpu;
+ smp_mb(); /* Pairs with barriers in rcu_prepare_cpu(). */
+ return ret;
+}
#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_get_max_cpu())->mynode; \
+ (rnp)++)
/*
* Do a breadth-first scan of the non-leaf rcu_node structures for the
--
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