rcu: Rename rcu_check_callbacks() to rcu_sched_clock_irq()

The name rcu_check_callbacks() arguably made sense back in the early
2000s when RCU was quite a bit simpler than it is today, but it has
become quite misleading, especially with the advent of dyntick-idle
and NO_HZ_FULL.  The rcu_check_callbacks() function is RCU's hook into
the scheduling-clock interrupt, and is now but one of many ways that
callbacks get promoted to invocable state.

This commit therefore changes the name to rcu_sched_clock_irq(),
which is the same number of characters and clearly indicates this
function's relation to the rest of the Linux kernel.  In addition, for
the sake of consistency, rcu_flavor_check_callbacks() is also renamed
to rcu_flavor_sched_clock_irq().

While in the area, the header comments for both functions are reworked.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html

@@ -485,7 +485,7 @@ <h4><a name="Self-Reported Quiescent States">
 noted by <tt>rcu_node_context_switch()</tt> on the left.
 On the other hand, if the CPU takes a scheduler-clock interrupt
 while executing in usermode, a quiescent state will be noted by
-<tt>rcu_check_callbacks()</tt> on the right.
+<tt>rcu_sched_clock_irq()</tt> on the right.
 Either way, the passage through a quiescent state will be noted
 in a per-CPU variable.
 
@@ -651,7 +651,7 @@ <h4><a name="Callback Invocation">Callback Invocation</a></h4>
 These callbacks are identified by <tt>rcu_advance_cbs()</tt>,
 which is usually invoked by <tt>__note_gp_changes()</tt>.
 As shown in the diagram below, this invocation can be triggered by
-the scheduling-clock interrupt (<tt>rcu_check_callbacks()</tt> on
+the scheduling-clock interrupt (<tt>rcu_sched_clock_irq()</tt> on
 the left) or by idle entry (<tt>rcu_cleanup_after_idle()</tt> on
 the right, but only for kernels build with
 <tt>CONFIG_RCU_FAST_NO_HZ=y</tt>).

include/linux/rcupdate.h

@@ -89,7 +89,7 @@ static inline int rcu_preempt_depth(void)
 /* Internal to kernel */
 void rcu_init(void);
 extern int rcu_scheduler_active __read_mostly;
-void rcu_check_callbacks(int user);
+void rcu_sched_clock_irq(int user);
 void rcu_report_dead(unsigned int cpu);
 void rcutree_migrate_callbacks(int cpu);
 

kernel/rcu/tiny.c

@@ -76,7 +76,7 @@ void rcu_qs(void)
  * be called from hardirq context.  It is normally called from the
  * scheduling-clock interrupt.
  */
-void rcu_check_callbacks(int user)
+void rcu_sched_clock_irq(int user)
 {
 	if (user) {
 		rcu_qs();

kernel/rcu/tree.c

@@ -1139,7 +1139,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 	}
 
 	/*
-	 * NO_HZ_FULL CPUs can run in-kernel without rcu_check_callbacks!
+	 * NO_HZ_FULL CPUs can run in-kernel without rcu_sched_clock_irq!
 	 * The above code handles this, but only for straight cond_resched().
 	 * And some in-kernel loops check need_resched() before calling
 	 * cond_resched(), which defeats the above code for CPUs that are
@@ -2532,14 +2532,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
 }
 
 /*
- * Check to see if this CPU is in a non-context-switch quiescent state
- * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
- * Also schedule RCU core processing.
- *
- * This function must be called from hardirq context.  It is normally
- * invoked from the scheduling-clock interrupt.
+ * This function is invoked from each scheduling-clock interrupt,
+ * and checks to see if this CPU is in a non-context-switch quiescent
+ * state, for example, user mode or idle loop.  It also schedules RCU
+ * core processing.  If the current grace period has gone on too long,
+ * it will ask the scheduler to manufacture a context switch for the sole
+ * purpose of providing a providing the needed quiescent state.
  */
-void rcu_check_callbacks(int user)
+void rcu_sched_clock_irq(int user)
 {
 	trace_rcu_utilization(TPS("Start scheduler-tick"));
 	raw_cpu_inc(rcu_data.ticks_this_gp);
@@ -2552,7 +2552,7 @@ void rcu_check_callbacks(int user)
 		}
 		__this_cpu_write(rcu_data.rcu_urgent_qs, false);
 	}
-	rcu_flavor_check_callbacks(user);
+	rcu_flavor_sched_clock_irq(user);
 	if (rcu_pending())
 		invoke_rcu_core();
 

kernel/rcu/tree.h

@@ -417,7 +417,7 @@ static void rcu_print_detail_task_stall(void);
 static int rcu_print_task_stall(struct rcu_node *rnp);
 static int rcu_print_task_exp_stall(struct rcu_node *rnp);
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp);
-static void rcu_flavor_check_callbacks(int user);
+static void rcu_flavor_sched_clock_irq(int user);
 void call_rcu(struct rcu_head *head, rcu_callback_t func);
 static void dump_blkd_tasks(struct rcu_node *rnp, int ncheck);
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);

kernel/rcu/tree_plugin.h

@@ -297,7 +297,7 @@ static void rcu_qs(void)
 				       __this_cpu_read(rcu_data.gp_seq),
 				       TPS("cpuqs"));
 		__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
-		barrier(); /* Coordinate with rcu_flavor_check_callbacks(). */
+		barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
 		current->rcu_read_unlock_special.b.need_qs = false;
 	}
 }
@@ -778,13 +778,13 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 }
 
 /*
- * Check for a quiescent state from the current CPU.  When a task blocks,
- * the task is recorded in the corresponding CPU's rcu_node structure,
- * which is checked elsewhere.
- *
- * Caller must disable hard irqs.
+ * Check for a quiescent state from the current CPU, including voluntary
+ * context switches for Tasks RCU.  When a task blocks, the task is
+ * recorded in the corresponding CPU's rcu_node structure, which is checked
+ * elsewhere, hence this function need only check for quiescent states
+ * related to the current CPU, not to those related to tasks.
  */
-static void rcu_flavor_check_callbacks(int user)
+static void rcu_flavor_sched_clock_irq(int user)
 {
 	struct task_struct *t = current;
 
@@ -1030,14 +1030,10 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 }
 
 /*
- * Check to see if this CPU is in a non-context-switch quiescent state
- * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
- * Also schedule RCU core processing.
- *
- * This function must be called from hardirq context.  It is normally
- * invoked from the scheduling-clock interrupt.
+ * Check to see if this CPU is in a non-context-switch quiescent state,
+ * namely user mode and idle loop.
  */
-static void rcu_flavor_check_callbacks(int user)
+static void rcu_flavor_sched_clock_irq(int user)
 {
 	if (user || rcu_is_cpu_rrupt_from_idle()) {
 

kernel/time/timer.c

@@ -1632,7 +1632,7 @@ void update_process_times(int user_tick)
 	/* Note: this timer irq context must be accounted for as well. */
 	account_process_tick(p, user_tick);
 	run_local_timers();
-	rcu_check_callbacks(user_tick);
+	rcu_sched_clock_irq(user_tick);
 #ifdef CONFIG_IRQ_WORK
 	if (in_irq())
 		irq_work_tick();