rcu: Move RCU grace-period initialization into a kthread

As the first step towards allowing grace-period initialization to be
preemptible, this commit moves the RCU grace-period initialization
into its own kthread.  This is needed to keep large-system scheduling
latency at reasonable levels.

Also change raw_spin_lock_irqsave() to raw_spin_lock_irq() as suggested
by Peter Zijlstra in review comments.

Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

1 files changed, 126 insertions, 64 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index f7bcd9e6c054..4792f1642bf2 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1042,6 +1042,102 @@ rcu_start_gp_per_cpu(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat
 }
 
 /*
+ * Body of kthread that handles grace periods.
+ */
+static int rcu_gp_kthread(void *arg)
+{
+        struct rcu_data *rdp;
+        struct rcu_node *rnp;
+        struct rcu_state *rsp = arg;
+
+        for (;;) {
+
+                /* Handle grace-period start. */
+                rnp = rcu_get_root(rsp);
+                for (;;) {
+                        wait_event_interruptible(rsp->gp_wq, rsp->gp_flags);
+                        if (rsp->gp_flags)
+                                break;
+                        flush_signals(current);
+                }
+                raw_spin_lock_irq(&rnp->lock);
+                rsp->gp_flags = 0;
+                rdp = this_cpu_ptr(rsp->rda);
+
+                if (rcu_gp_in_progress(rsp)) {
+                        /*
+                         * A grace period is already in progress, so
+                         * don't start another one.
+                         */
+                        raw_spin_unlock_irq(&rnp->lock);
+                        continue;
+                }
+
+                if (rsp->fqs_active) {
+                        /*
+                         * We need a grace period, but force_quiescent_state()
+                         * is running.  Tell it to start one on our behalf.
+                         */
+                        rsp->fqs_need_gp = 1;
+                        raw_spin_unlock_irq(&rnp->lock);
+                        continue;
+                }
+
+                /* Advance to a new grace period and initialize state. */
+                rsp->gpnum++;
+                trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
+                WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
+                rsp->fqs_state = RCU_GP_INIT; /* Stop force_quiescent_state. */
+                rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
+                record_gp_stall_check_time(rsp);
+                raw_spin_unlock(&rnp->lock);  /* leave irqs disabled. */
+
+                /* Exclude any concurrent CPU-hotplug operations. */
+                raw_spin_lock(&rsp->onofflock);  /* irqs already disabled. */
+
+                /*
+                 * Set the quiescent-state-needed bits in all the rcu_node
+                 * structures for all currently online CPUs in breadth-first
+                 * order, starting from the root rcu_node structure.
+                 * This operation relies on the layout of the hierarchy
+                 * within the rsp->node[] array.  Note that other CPUs will
+                 * access only the leaves of the hierarchy, which still
+                 * indicate that no grace period is in progress, at least
+                 * until the corresponding leaf node has been initialized.
+                 * In addition, we have excluded CPU-hotplug operations.
+                 *
+                 * Note that the grace period cannot complete until
+                 * we finish the initialization process, as there will
+                 * be at least one qsmask bit set in the root node until
+                 * that time, namely the one corresponding to this CPU,
+                 * due to the fact that we have irqs disabled.
+                 */
+                rcu_for_each_node_breadth_first(rsp, rnp) {
+                        raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+                        rcu_preempt_check_blocked_tasks(rnp);
+                        rnp->qsmask = rnp->qsmaskinit;
+                        rnp->gpnum = rsp->gpnum;
+                        rnp->completed = rsp->completed;
+                        if (rnp == rdp->mynode)
+                                rcu_start_gp_per_cpu(rsp, rnp, rdp);
+                        rcu_preempt_boost_start_gp(rnp);
+                        trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
+                                                    rnp->level, rnp->grplo,
+                                                    rnp->grphi, rnp->qsmask);
+                        raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+                }
+
+                rnp = rcu_get_root(rsp);
+                raw_spin_lock(&rnp->lock); /* irqs already disabled. */
+                /* force_quiescent_state() now OK. */
+                rsp->fqs_state = RCU_SIGNAL_INIT;
+                raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */
+                raw_spin_unlock_irq(&rsp->onofflock);
+        }
+        return 0;
+}
+
+/*
  * Start a new RCU grace period if warranted, re-initializing the hierarchy
  * in preparation for detecting the next grace period.  The caller must hold
  * the root node's ->lock, which is released before return.  Hard irqs must
@@ -1058,77 +1154,20 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
         struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
         struct rcu_node *rnp = rcu_get_root(rsp);
 
-        if (!rcu_scheduler_fully_active ||
+        if (!rsp->gp_kthread ||
             !cpu_needs_another_gp(rsp, rdp)) {
                 /*
-                 * Either the scheduler hasn't yet spawned the first
-                 * non-idle task or this CPU does not need another
-                 * grace period.  Either way, don't start a new grace
-                 * period.
+                 * Either we have not yet spawned the grace-period
+                 * task or this CPU does not need another grace period.
+                 * Either way, don't start a new grace period.
                  */
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 return;
         }
 
-        if (rsp->fqs_active) {
-                /*
-                 * This CPU needs a grace period, but force_quiescent_state()
-                 * is running.  Tell it to start one on this CPU's behalf.
-                 */
-                rsp->fqs_need_gp = 1;
-                raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                return;
-        }
-
-        /* Advance to a new grace period and initialize state. */
-        rsp->gpnum++;
-        trace_rcu_grace_period(rsp->name, rsp->gpnum, "start");
-        WARN_ON_ONCE(rsp->fqs_state == RCU_GP_INIT);
-        rsp->fqs_state = RCU_GP_INIT; /* Hold off force_quiescent_state. */
-        rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
-        record_gp_stall_check_time(rsp);
-        raw_spin_unlock(&rnp->lock);  /* leave irqs disabled. */
-
-        /* Exclude any concurrent CPU-hotplug operations. */
-        raw_spin_lock(&rsp->onofflock);  /* irqs already disabled. */
-
-        /*
-         * Set the quiescent-state-needed bits in all the rcu_node
-         * structures for all currently online CPUs in breadth-first
-         * order, starting from the root rcu_node structure.  This
-         * operation relies on the layout of the hierarchy within the
-         * rsp->node[] array.  Note that other CPUs will access only
-         * the leaves of the hierarchy, which still indicate that no
-         * grace period is in progress, at least until the corresponding
-         * leaf node has been initialized.  In addition, we have excluded
-         * CPU-hotplug operations.
-         *
-         * Note that the grace period cannot complete until we finish
-         * the initialization process, as there will be at least one
-         * qsmask bit set in the root node until that time, namely the
-         * one corresponding to this CPU, due to the fact that we have
-         * irqs disabled.
-         */
-        rcu_for_each_node_breadth_first(rsp, rnp) {
-                raw_spin_lock(&rnp->lock);      /* irqs already disabled. */
-                rcu_preempt_check_blocked_tasks(rnp);
-                rnp->qsmask = rnp->qsmaskinit;
-                rnp->gpnum = rsp->gpnum;
-                rnp->completed = rsp->completed;
-                if (rnp == rdp->mynode)
-                        rcu_start_gp_per_cpu(rsp, rnp, rdp);
-                rcu_preempt_boost_start_gp(rnp);
-                trace_rcu_grace_period_init(rsp->name, rnp->gpnum,
-                                            rnp->level, rnp->grplo,
-                                            rnp->grphi, rnp->qsmask);
-                raw_spin_unlock(&rnp->lock);    /* irqs remain disabled. */
-        }
-
-        rnp = rcu_get_root(rsp);
-        raw_spin_lock(&rnp->lock);              /* irqs already disabled. */
-        rsp->fqs_state = RCU_SIGNAL_INIT; /* force_quiescent_state now OK. */
-        raw_spin_unlock(&rnp->lock);            /* irqs remain disabled. */
-        raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+        rsp->gp_flags = 1;
+        raw_spin_unlock_irqrestore(&rnp->lock, flags);
+        wake_up(&rsp->gp_wq);
 }
 
 /*
@@ -2629,6 +2668,28 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 }
 
 /*
+ * Spawn the kthread that handles this RCU flavor's grace periods.
+ */
+static int __init rcu_spawn_gp_kthread(void)
+{
+        unsigned long flags;
+        struct rcu_node *rnp;
+        struct rcu_state *rsp;
+        struct task_struct *t;
+
+        for_each_rcu_flavor(rsp) {
+                t = kthread_run(rcu_gp_kthread, rsp, rsp->name);
+                BUG_ON(IS_ERR(t));
+                rnp = rcu_get_root(rsp);
+                raw_spin_lock_irqsave(&rnp->lock, flags);
+                rsp->gp_kthread = t;
+                raw_spin_unlock_irqrestore(&rnp->lock, flags);
+        }
+        return 0;
+}
+early_initcall(rcu_spawn_gp_kthread);
+
+/*
  * This function is invoked towards the end of the scheduler's initialization
  * process.  Before this is called, the idle task might contain
  * RCU read-side critical sections (during which time, this idle
@@ -2729,6 +2790,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
         }
 
         rsp->rda = rda;
+        init_waitqueue_head(&rsp->gp_wq);
         rnp = rsp->level[rcu_num_lvls - 1];
         for_each_possible_cpu(i) {
                 while (i > rnp->grphi)