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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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