rcu/segcblist: Prevent useless GP start if no CBs to accelerate

The rcu_segcblist_accelerate() function returns true iff it is necessary
to request another grace period.  A tracing session showed that this
function unnecessarily requests grace periods.

For example, consider the following sequence of events:
1. Callbacks are queued only on the NEXT segment of CPU A's callback list.
2. CPU A runs RCU_SOFTIRQ, accelerating these callbacks from NEXT to WAIT.
3. Thus rcu_segcblist_accelerate() returns true, requesting grace period N.
4. RCU's grace-period kthread wakes up on CPU B and starts grace period N.
4. CPU A notices the new grace period and invokes RCU_SOFTIRQ.
5. CPU A's RCU_SOFTIRQ again invokes rcu_segcblist_accelerate(), but
   there are no new callbacks.  However, rcu_segcblist_accelerate()
   nevertheless (uselessly) requests a new grace period N+1.

This extra grace period results in additional lock contention and also
additional wakeups, all for no good reason.

This commit therefore adds a check to rcu_segcblist_accelerate() that
prevents the return of true when there are no new callbacks.

This change reduces the number of grace periods (GPs) and wakeups in each
of eleven five-second rcutorture runs as follows:

+----+-------------------+-------------------+
| #  | Number of GPs     | Number of Wakeups |
+====+=========+=========+=========+=========+
| 1  | With    | Without | With    | Without |
+----+---------+---------+---------+---------+
| 2  |      75 |      89 |     113 |     119 |
+----+---------+---------+---------+---------+
| 3  |      62 |      91 |     105 |     123 |
+----+---------+---------+---------+---------+
| 4  |      60 |      79 |      98 |     110 |
+----+---------+---------+---------+---------+
| 5  |      63 |      79 |      99 |     112 |
+----+---------+---------+---------+---------+
| 6  |      57 |      89 |      96 |     123 |
+----+---------+---------+---------+---------+
| 7  |      64 |      85 |      97 |     118 |
+----+---------+---------+---------+---------+
| 8  |      58 |      83 |      98 |     113 |
+----+---------+---------+---------+---------+
| 9  |      57 |      77 |      89 |     104 |
+----+---------+---------+---------+---------+
| 10 |      66 |      82 |      98 |     119 |
+----+---------+---------+---------+---------+
| 11 |      52 |      82 |      83 |     117 |
+----+---------+---------+---------+---------+

The reduction in the number of wakeups ranges from 5% to 40%.

Cc: urezki@gmail.com
[ paulmck: Rework commit log and comment. ]
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/rcu_segcblist.c

@@ -475,8 +475,16 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
 	 * Also advance to the oldest segment of callbacks whose
 	 * ->gp_seq[] completion is at or after that passed in via "seq",
 	 * skipping any empty segments.
+	 *
+	 * Note that segment "i" (and any lower-numbered segments
+	 * containing older callbacks) will be unaffected, and their
+	 * grace-period numbers remain unchanged.  For example, if i ==
+	 * WAIT_TAIL, then neither WAIT_TAIL nor DONE_TAIL will be touched.
+	 * Instead, the CBs in NEXT_TAIL will be merged with those in
+	 * NEXT_READY_TAIL and the grace-period number of NEXT_READY_TAIL
+	 * would be updated.  NEXT_TAIL would then be empty.
 	 */
-	if (++i >= RCU_NEXT_TAIL)
+	if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL)
 		return false;
 
 	/*