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    SubjectRe: [PATCH RFC] rcu: Limit GP initialization to CPUs that have been online
    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?

    >
    > 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>
    >
    > 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..afdf410 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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