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SubjectRe: [PATCH RFC] rcu: Limit GP initialization to CPUs that have been online
On Thu, Mar 15, 2012 at 11:23:14AM -0700, Paul E. McKenney wrote:
> On Thu, Mar 15, 2012 at 12:58:57PM -0500, Dimitri Sivanich wrote:
> > On Wed, Mar 14, 2012 at 09:56:57AM -0700, Paul E. McKenney wrote:
> > > On Wed, Mar 14, 2012 at 08:17:17AM -0700, Paul E. McKenney wrote:
> > > > On Wed, Mar 14, 2012 at 08:08:01AM -0500, Dimitri Sivanich wrote:
> > > > > On Wed, Mar 14, 2012 at 01:40:41PM +0100, Mike Galbraith wrote:
> > > > > > On Wed, 2012-03-14 at 10:24 +0100, Mike Galbraith wrote:
> > > > > > > On Tue, 2012-03-13 at 17:24 -0700, Paul E. McKenney wrote:
> > > > > > > > The following builds, but is only very lightly tested. Probably full
> > > > > > > > of bug, especially when exercising CPU hotplug.
> > > > > > >
> > > > > > > You didn't say RFT, but...
> > > > > > >
> > > > > > > To beat on this in a rotund 3.0 kernel, the equivalent patch would be
> > > > > > > the below? My box may well answer that before you can.. hope not ;-)
> > > > > >
> > > > > > (Darn, it did. Box says boot stall with virgin patch in tip too though.
> > > > > > Wedging it straight into 3.0 was perhaps a tad premature;)
> > > > >
> > > > > I saw the same thing with 3.3.0-rc7+ and virgin patch on UV. Boots fine without the patch.
> > > >
> > > > Right... Bozo here forgot to set the kernel parameters for large-system
> > > > emulation during testing. Apologies for the busted patch, will fix.
> > > >
> > > > And thank you both for the testing!!!
> > > >
> > > > Hey, at least I labeled it "RFC". ;-)
> > >
> > > Does the following work better? It does pass my fake-big-system tests
> > > (more testing in the works).
> >
> > This one stalls for me at the same place the other one did. Once again,
> > if I remove the patch and rebuild, it boots just fine.
> >
> > Is there some debug/trace information that you would like me to provide?
>
> Very strange.
>
> Could you please send your dmesg and .config?

Hmmm... Memory ordering could be a problem, though in that case I would
have expected the hand during the onlining process. However, the memory
ordering does need to be cleaned up in any case, please see below.

Thanx, Paul

------------------------------------------------------------------------
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@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8269656..7247fa8 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. */
}

/*
@@ -2447,6 +2452,7 @@ 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);
@@ -2469,6 +2475,20 @@ 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.
+ * 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;
@@ -2502,6 +2522,11 @@ 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) {
+ smp_mb(); /* Initialization before rcu_max_cpu assignment. */
+ rcu_max_cpu = cpu;
+ smp_mb(); /* rcu_max_cpu assignment before later uses. */
+ }
}

/*
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 1e49c56..772df1c 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -192,11 +192,23 @@ 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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