[PATCH RFC] cpu: No more __stop_machine() in _cpu_down()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

The _cpu_down() function invoked as part of the CPU-hotplug offlining
process currently invokes __stop_machine(), which is slow and inflicts
substantial real-time latencies on the entire system.  This patch
substitutes stop_cpus() for __stop_machine() in order to improve
both performance and real-time latency.

This is currently unsafe, because there are a number of uses of
preempt_disable() that are intended to block CPU-hotplug offlining.
These will be fixed, but in the meantime, this commit is one way to help
locate them.  It nevertheless passes light rcutorture/hotplug stress
testing.  Meaning that we should not be relying on pure testing to find
places where people are relying on preemption disabling to block CPUs
from going offline.  ;-)

Not-yet-signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>

diff --git a/kernel/cpu.c b/kernel/cpu.c
index a4eb522..47e63a0 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -243,13 +243,18 @@ static int __ref take_cpu_down(void *_param)
 {
 	struct take_cpu_down_param *param = _param;
 	int err;
+	unsigned long flags;
 
 	/* Ensure this CPU doesn't handle any more interrupts. */
+	local_irq_save(flags);
 	err = __cpu_disable();
-	if (err < 0)
+	if (err < 0) {
+		local_irq_restore(flags);
 		return err;
+	}
 
 	cpu_notify(CPU_DYING | param->mod, param->hcpu);
+	local_irq_restore(flags);
 	return 0;
 }
 
@@ -281,7 +286,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 		goto out_release;
 	}
 
-	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
+	err = stop_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);
 	if (err) {
 		/* CPU didn't die: tell everyone.  Can't complain. */
 		cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);