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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2009-09-13 18:15:10 +0200
committerIngo Molnar <mingo@elte.hu>2009-09-18 00:06:33 +0200
commitc3422bea5f09b0e85704f51f2b01271630b8940b (patch)
treefc4a18241b0f7f5d71211f6787428197769cb9f5 /kernel
parentrcu: Add debug checks to TREE_PREEMPT_RCU for premature grace periods (diff)
downloadlinux-c3422bea5f09b0e85704f51f2b01271630b8940b.tar.xz
linux-c3422bea5f09b0e85704f51f2b01271630b8940b.zip
rcu: Simplify rcu_read_unlock_special() quiescent-state accounting
The earlier approach required two scheduling-clock ticks to note an preemptable-RCU quiescent state in the situation in which the scheduling-clock interrupt is unlucky enough to always interrupt an RCU read-side critical section. With this change, the quiescent state is instead noted by the outermost rcu_read_unlock() immediately following the first scheduling-clock tick, or, alternatively, by the first subsequent context switch. Therefore, this change also speeds up grace periods. Suggested-by: Josh Triplett <josh@joshtriplett.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu LKML-Reference: <12528585111945-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/rcutree.c15
-rw-r--r--kernel/rcutree_plugin.h54
2 files changed, 32 insertions, 37 deletions
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e9a4ae94647f..6c99553e9f15 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -107,27 +107,23 @@ static void __cpuinit rcu_init_percpu_data(int cpu, struct rcu_state *rsp,
*/
void rcu_sched_qs(int cpu)
{
- unsigned long flags;
struct rcu_data *rdp;
- local_irq_save(flags);
rdp = &per_cpu(rcu_sched_data, cpu);
- rdp->passed_quiesc = 1;
rdp->passed_quiesc_completed = rdp->completed;
- rcu_preempt_qs(cpu);
- local_irq_restore(flags);
+ barrier();
+ rdp->passed_quiesc = 1;
+ rcu_preempt_note_context_switch(cpu);
}
void rcu_bh_qs(int cpu)
{
- unsigned long flags;
struct rcu_data *rdp;
- local_irq_save(flags);
rdp = &per_cpu(rcu_bh_data, cpu);
- rdp->passed_quiesc = 1;
rdp->passed_quiesc_completed = rdp->completed;
- local_irq_restore(flags);
+ barrier();
+ rdp->passed_quiesc = 1;
}
#ifdef CONFIG_NO_HZ
@@ -615,6 +611,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
/* Advance to a new grace period and initialize state. */
rsp->gpnum++;
+ WARN_ON_ONCE(rsp->signaled == RCU_GP_INIT);
rsp->signaled = RCU_GP_INIT; /* Hold off force_quiescent_state. */
rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
record_gp_stall_check_time(rsp);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index b8e4b0384f00..c9616e48379b 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -64,34 +64,42 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
* not in a quiescent state. There might be any number of tasks blocked
* while in an RCU read-side critical section.
*/
-static void rcu_preempt_qs_record(int cpu)
+static void rcu_preempt_qs(int cpu)
{
struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu);
- rdp->passed_quiesc = 1;
rdp->passed_quiesc_completed = rdp->completed;
+ barrier();
+ rdp->passed_quiesc = 1;
}
/*
- * We have entered the scheduler or are between softirqs in ksoftirqd.
- * If we are in an RCU read-side critical section, we need to reflect
- * that in the state of the rcu_node structure corresponding to this CPU.
- * Caller must disable hardirqs.
+ * We have entered the scheduler, and the current task might soon be
+ * context-switched away from. If this task is in an RCU read-side
+ * critical section, we will no longer be able to rely on the CPU to
+ * record that fact, so we enqueue the task on the appropriate entry
+ * of the blocked_tasks[] array. The task will dequeue itself when
+ * it exits the outermost enclosing RCU read-side critical section.
+ * Therefore, the current grace period cannot be permitted to complete
+ * until the blocked_tasks[] entry indexed by the low-order bit of
+ * rnp->gpnum empties.
+ *
+ * Caller must disable preemption.
*/
-static void rcu_preempt_qs(int cpu)
+static void rcu_preempt_note_context_switch(int cpu)
{
struct task_struct *t = current;
+ unsigned long flags;
int phase;
struct rcu_data *rdp;
struct rcu_node *rnp;
if (t->rcu_read_lock_nesting &&
(t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
- WARN_ON_ONCE(cpu != smp_processor_id());
/* Possibly blocking in an RCU read-side critical section. */
rdp = rcu_preempt_state.rda[cpu];
rnp = rdp->mynode;
- spin_lock(&rnp->lock);
+ spin_lock_irqsave(&rnp->lock, flags);
t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
t->rcu_blocked_node = rnp;
@@ -112,7 +120,7 @@ static void rcu_preempt_qs(int cpu)
phase = !(rnp->qsmask & rdp->grpmask) ^ (rnp->gpnum & 0x1);
list_add(&t->rcu_node_entry, &rnp->blocked_tasks[phase]);
smp_mb(); /* Ensure later ctxt swtch seen after above. */
- spin_unlock(&rnp->lock);
+ spin_unlock_irqrestore(&rnp->lock, flags);
}
/*
@@ -124,9 +132,8 @@ static void rcu_preempt_qs(int cpu)
* grace period, then the fact that the task has been enqueued
* means that we continue to block the current grace period.
*/
- rcu_preempt_qs_record(cpu);
- t->rcu_read_unlock_special &= ~(RCU_READ_UNLOCK_NEED_QS |
- RCU_READ_UNLOCK_GOT_QS);
+ rcu_preempt_qs(cpu);
+ t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
}
/*
@@ -162,7 +169,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
special = t->rcu_read_unlock_special;
if (special & RCU_READ_UNLOCK_NEED_QS) {
t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
- t->rcu_read_unlock_special |= RCU_READ_UNLOCK_GOT_QS;
+ rcu_preempt_qs(smp_processor_id());
}
/* Hardware IRQ handlers cannot block. */
@@ -199,9 +206,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
*/
if (!empty && rnp->qsmask == 0 &&
list_empty(&rnp->blocked_tasks[rnp->gpnum & 0x1])) {
- t->rcu_read_unlock_special &=
- ~(RCU_READ_UNLOCK_NEED_QS |
- RCU_READ_UNLOCK_GOT_QS);
+ t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
if (rnp->parent == NULL) {
/* Only one rcu_node in the tree. */
cpu_quiet_msk_finish(&rcu_preempt_state, flags);
@@ -352,19 +357,12 @@ static void rcu_preempt_check_callbacks(int cpu)
struct task_struct *t = current;
if (t->rcu_read_lock_nesting == 0) {
- t->rcu_read_unlock_special &=
- ~(RCU_READ_UNLOCK_NEED_QS | RCU_READ_UNLOCK_GOT_QS);
- rcu_preempt_qs_record(cpu);
+ t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;
+ rcu_preempt_qs(cpu);
return;
}
if (per_cpu(rcu_preempt_data, cpu).qs_pending) {
- if (t->rcu_read_unlock_special & RCU_READ_UNLOCK_GOT_QS) {
- rcu_preempt_qs_record(cpu);
- t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_GOT_QS;
- } else if (!(t->rcu_read_unlock_special &
- RCU_READ_UNLOCK_NEED_QS)) {
- t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
- }
+ t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
}
}
@@ -451,7 +449,7 @@ EXPORT_SYMBOL_GPL(rcu_batches_completed);
* Because preemptable RCU does not exist, we never have to check for
* CPUs being in quiescent states.
*/
-static void rcu_preempt_qs(int cpu)
+static void rcu_preempt_note_context_switch(int cpu)
{
}