[PATCH 1/7] rcu/segcblist: Prevent useless GP start if no CBs to accelerate

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

rcu_segcblist_accelerate() returns true if a GP is to be
started/requested and false if not. During tracing, I found that it is
asking that GPs be requested

The exact flow seems to be something like:
1. Callbacks are queued on CPU A - into the NEXT list.
2. softirq runs on CPU A, accelerate all CBs from NEXT->WAIT and request a GP X.
3. GP thread wakes up on another CPU, starts the GP X and requests QS from CPU A.
4. CPU A's softirq runs again presumably because of #3.
5. CPU A's softirq now does acceleration again, this time no CBs are
   accelerated since last attempt, but it still requests GP X+1 which
   could be useless.

The fix is, prevent the useless GP start if we detect no CBs are there
to accelerate.

With this, we have the following improvement in short runs of
rcutorture (5 seconds each):
+----+-------------------+-------------------+
| #  | 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 |
+----+---------+---------+---------+---------+

Cc: urezki@gmail.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/rcu_segcblist.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 9a0f66133b4b3..4782cf17bf4f9 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -475,8 +475,15 @@ 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 "i" is the youngest segment of the list after which
+	 * any older segments than "i" would not be mutated or assigned
+	 * GPs. For example, if i == WAIT_TAIL, then neither WAIT_TAIL,
+	 * nor DONE_TAIL will be touched. Only CBs in NEXT_TAIL will be
+	 * merged with NEXT_READY_TAIL and the GP numbers of both of
+	 * them would be updated.
 	 */
-	if (++i >= RCU_NEXT_TAIL)
+	if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL)
 		return false;
 
 	/*
-- 
2.27.0.111.gc72c7da667-goog