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>

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)
 {
 }