rcu: Add callback-free CPUs

RCU callback execution can add significant OS jitter and also can
degrade both scheduling latency and, in asymmetric multiprocessors,
energy efficiency.  This commit therefore adds the ability for selected
CPUs ("rcu_nocbs=" boot parameter) to have their callbacks offloaded
to kthreads.  If the "rcu_nocb_poll" boot parameter is also specified,
these kthreads will do polling, removing the need for the offloaded
CPUs to do wakeups.  At least one CPU must be doing normal callback
processing: currently CPU 0 cannot be selected as a no-CBs CPU.
In addition, attempts to offline the last normal-CBs CPU will fail.

This feature was inspired by Jim Houston's and Joe Korty's JRCU, and
this commit includes fixes to problems located by Fengguang Wu's
kbuild test robot.

[ paulmck: Added gfp.h include file as suggested by Fengguang Wu. ]

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

1 files changed, 6 insertions, 1 deletions

diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index f9512687a6e5..3189f9aa3e84 100644
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -113,6 +113,8 @@ static char convert_kthread_status(unsigned int kthread_status)
 
 static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
 {
+        long ql, qll;
+
         if (!rdp->beenonline)
                 return;
         seq_printf(m, "%3d%cc=%ld g=%ld pq=%d qp=%d",
@@ -126,8 +128,11 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
                    rdp->dynticks->dynticks_nmi_nesting,
                    rdp->dynticks_fqs);
         seq_printf(m, " of=%lu", rdp->offline_fqs);
+        rcu_nocb_q_lengths(rdp, &ql, &qll);
+        qll += rdp->qlen_lazy;
+        ql += rdp->qlen;
         seq_printf(m, " ql=%ld/%ld qs=%c%c%c%c",
-                   rdp->qlen_lazy, rdp->qlen,
+                   qll, ql,
                    ".N"[rdp->nxttail[RCU_NEXT_READY_TAIL] !=
                         rdp->nxttail[RCU_NEXT_TAIL]],
                    ".R"[rdp->nxttail[RCU_WAIT_TAIL] !=