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Subject[PATCH tip/core/rcu 18/22] rcu: Move RCU grace-period initialization into a kthread
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

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.

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>
---
kernel/rcutree.c | 191 ++++++++++++++++++++++++++++++++++++------------------
kernel/rcutree.h | 3 +
2 files changed, 130 insertions(+), 64 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1b0e441..faf4860 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1030,6 +1030,103 @@ 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)
+{
+ unsigned long flags;
+ 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_irqsave(&rnp->lock, flags);
+ 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_irqrestore(&rnp->lock, flags);
+ 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_irqrestore(&rnp->lock, flags);
+ 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_irqrestore(&rsp->onofflock, flags);
+ }
+ 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
@@ -1046,77 +1143,20 @@ 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.
- */
- 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.
+ * 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.
*/
- 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);
}

/*
@@ -2615,6 +2655,28 @@ 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
@@ -2715,6 +2777,7 @@ 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)
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 138fb33..c7c31c2 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -370,6 +370,9 @@ struct rcu_state {
u8 boost; /* Subject to priority boost. */
unsigned long gpnum; /* Current gp number. */
unsigned long completed; /* # of last completed gp. */
+ struct task_struct *gp_kthread; /* Task for grace periods. */
+ wait_queue_head_t gp_wq; /* Where GP task waits. */
+ int gp_flags; /* Commands for GP task. */

/* End of fields guarded by root rcu_node's lock. */

--
1.7.8


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