rcu: Avoid irq disable in rcu_cpu_kthread

[Christoph Lameter]

Once we have the per cpu patchset merged we could do the following [it
even works without that patchset but the __this_cpu ops will not do
preemption checks]. Would this work?


Subject: rcu: Avoid irq disable in rcu_cpu_kthread

The use of this_cpu ops avoids numerous address calculations
and allows to avoid the irq enable/disable sequence through a
low latency non locking this_cpu_xchg.

Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

Index: linux/kernel/rcu/tree_plugin.h
===================================================================
--- linux.orig/kernel/rcu/tree_plugin.h	2013-12-03 11:32:23.322999660 -0600
+++ linux/kernel/rcu/tree_plugin.h	2013-12-03 11:32:23.312999941 -0600
@@ -1417,33 +1417,29 @@ static int rcu_cpu_kthread_should_run(un
  */
 static void rcu_cpu_kthread(unsigned int cpu)
 {
-	unsigned int *statusp = this_cpu_ptr(&rcu_cpu_kthread_status);
-	char work, *workp = this_cpu_ptr(&rcu_cpu_has_work);
+	char work;
 	int spincnt;

 	for (spincnt = 0; spincnt < 10; spincnt++) {
 		trace_rcu_utilization(TPS("Start CPU kthread@rcu_wait"));
 		local_bh_disable();
-		*statusp = RCU_KTHREAD_RUNNING;
-		this_cpu_inc(rcu_cpu_kthread_loops);
-		local_irq_disable();
-		work = *workp;
-		*workp = 0;
-		local_irq_enable();
+		__this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_RUNNING);
+		__this_cpu_inc(rcu_cpu_kthread_loops);
+		work = this_cpu_xchg(rcu_cpu_has_work, 0);
 		if (work)
 			rcu_kthread_do_work();
 		local_bh_enable();
-		if (*workp == 0) {
+		if (__this_cpu_read(rcu_cpu_has_work) == 0) {
 			trace_rcu_utilization(TPS("End CPU kthread@rcu_wait"));
-			*statusp = RCU_KTHREAD_WAITING;
+			__this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_WAITING);
 			return;
 		}
 	}
-	*statusp = RCU_KTHREAD_YIELDING;
+	__this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_YIELDING);
 	trace_rcu_utilization(TPS("Start CPU kthread@rcu_yield"));
 	schedule_timeout_interruptible(2);
 	trace_rcu_utilization(TPS("End CPU kthread@rcu_yield"));
-	*statusp = RCU_KTHREAD_WAITING;
+	__this_cpu_write(rcu_cpu_kthread_status, RCU_KTHREAD_WAITING);
 }

 /*
--
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