Messages in this thread Patch in this message | | | Date | Tue, 13 Mar 2012 17:24:14 -0700 | From | "Paul E. McKenney" <> | Subject | [PATCH RFC] rcu: Limit GP initialization to CPUs that have been online |
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The following builds, but is only very lightly tested. Probably full of bug, especially when exercising CPU hotplug.
Thanx, Paul
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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..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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