[PATCH v6 44/46] cpu: No more __stop_machine() in _cpu_down()

[Srivatsa S. Bhat]

From: Paul E. McKenney <paul.mckenney at linaro.org>

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_one_cpu() for __stop_machine() in order to improve
both performance and real-time latency.

There were a number of uses of preempt_disable() or local_irq_disable()
that were intended to block CPU-hotplug offlining. These were fixed by
using get/put_online_cpus_atomic(), which is the new synchronization
primitive to prevent CPU offline, while invoking from atomic context.

Signed-off-by: Paul E. McKenney <paul.mckenney at linaro.org>
Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
[ srivatsa.bhat at linux.vnet.ibm.com: Refer to the new sync primitives for
  readers (in the changelog); s/stop_cpus/stop_one_cpu and fix comment
  referring to stop_machine in the code]
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
---

 kernel/cpu.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 58dd1df..652fb53 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -337,7 +337,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 	}
 	smpboot_park_threads(cpu);
 
-	err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
+	err = stop_one_cpu(cpu, take_cpu_down, &tcd_param);
 	if (err) {
 		/* CPU didn't die: tell everyone.  Can't complain. */
 		smpboot_unpark_threads(cpu);
@@ -349,7 +349,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 	/*
 	 * The migration_call() CPU_DYING callback will have removed all
 	 * runnable tasks from the cpu, there's only the idle task left now
-	 * that the migration thread is done doing the stop_machine thing.
+	 * that the migration thread is done doing the stop_one_cpu() thing.
 	 *
 	 * Wait for the stop thread to go away.
 	 */