[PATCH 4/8] refscale: Add preempt_disable() readers

[Paul E. McKenney]

This commit adds refscale readers based on preempt_disable() and
preempt_enable() ("refscale.scale_type=preempt").  On my x86 laptop, these
are about 2.8ns.

Signed-off-by: Paul E. McKenney <paul...@kernel.org>
---
 kernel/rcu/refscale.c | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
index 1faed90231ab..ac9a0b6332ac 100644
--- a/kernel/rcu/refscale.c
+++ b/kernel/rcu/refscale.c
@@ -636,6 +636,37 @@ static const struct ref_scale_ops jiffies_ops = {
        .name           = "jiffies"
 };
 
+static void ref_preempt_section(const int nloops)
+{
+       int i;
+
+       migrate_disable();
+       for (i = nloops; i >= 0; i--) {
+               preempt_disable();
+               preempt_enable();
+       }
+       migrate_enable();
+}
+
+static void ref_preempt_delay_section(const int nloops, const int udl, const 
int ndl)
+{
+       int i;
+
+       migrate_disable();
+       for (i = nloops; i >= 0; i--) {
+               preempt_disable();
+               un_delay(udl, ndl);
+               preempt_enable();
+       }
+       migrate_enable();
+}
+
+static const struct ref_scale_ops preempt_ops = {
+       .readsection    = ref_preempt_section,
+       .delaysection   = ref_preempt_delay_section,
+       .name           = "preempt"
+};
+
 static void ref_bh_section(const int nloops)
 {
        int i;
@@ -1268,7 +1299,7 @@ ref_scale_init(void)
                &rcu_ops, &srcu_ops, &srcu_fast_ops, RCU_TRACE_OPS RCU_TASKS_OPS
                &refcnt_ops, &rwlock_ops, &rwsem_ops, &lock_ops, &lock_irq_ops,
                &acqrel_ops, &sched_clock_ops, &clock_ops, &jiffies_ops,
-               &bh_ops, &irq_ops, &irqsave_ops,
+               &preempt_ops, &bh_ops, &irq_ops, &irqsave_ops,
                &typesafe_ref_ops, &typesafe_lock_ops, &typesafe_seqlock_ops,
        };
 
-- 
2.40.1