rcu: Add tracing data to support queueing models

The current tracing data is not sufficient to deduce the average time
that a callback spends waiting for a grace period to end.  Add three
per-CPU counters recording the number of callbacks invoked (ci), the
number of callbacks orphaned (co), and the number of callbacks adopted
(ca).  Given the existing callback queue length (ql), the average wait
time in absence of CPU hotplug operations is ql/ci.  The units of wait
time will be in terms of the duration over which ci was measured.

In the presence of CPU hotplug operations, there is room for argument,
but ql/(ci-co+ca) won't steer you too far wrong.

Also fixes a typo called out by Lucas De Marchi <lucas.de.marchi@gmail.com>.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 42140a860bb9..e75073504a31 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1004,6 +1004,7 @@ static void rcu_send_cbs_to_orphanage(struct rcu_state *rsp)
         for (i = 0; i < RCU_NEXT_SIZE; i++)
                 rdp->nxttail[i] = &rdp->nxtlist;
         rsp->orphan_qlen += rdp->qlen;
+        rdp->n_cbs_orphaned += rdp->qlen;
         rdp->qlen = 0;
         raw_spin_unlock(&rsp->onofflock);  /* irqs remain disabled. */
 }
@@ -1025,6 +1026,7 @@ static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
         *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_list;
         rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_cbs_tail;
         rdp->qlen += rsp->orphan_qlen;
+        rdp->n_cbs_adopted += rsp->orphan_qlen;
         rsp->orphan_cbs_list = NULL;
         rsp->orphan_cbs_tail = &rsp->orphan_cbs_list;
         rsp->orphan_qlen = 0;
@@ -1156,6 +1158,7 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 
         /* Update count, and requeue any remaining callbacks. */
         rdp->qlen -= count;
+        rdp->n_cbs_invoked += count;
         if (list != NULL) {
                 *tail = rdp->nxtlist;
                 rdp->nxtlist = list;