linux-kernel - [PATCH tip/core/rcu 3/7] cpu: No more __stop_machine() in _cpu

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Date:	Tue, 30 Oct 2012 09:54:32 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, edumazet@...gle.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 3/7] cpu: No more __stop_machine() in _cpu_down()

From: "Paul E. McKenney" <paul.mckenney@...aro.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_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.

Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/cpu.c |    9 +++++++--
 1 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 42bd331..6b37ce0 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -247,13 +247,18 @@ 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;
 }
 
@@ -286,7 +291,7 @@ 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_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);
 	if (err) {
 		/* CPU didn't die: tell everyone.  Can't complain. */
 		smpboot_unpark_threads(cpu);
-- 
1.7.8

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