linux-kernel - [RFC PATCH v4 9/9] cpu: No more __stop_machine() in _cpu

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Message-ID: <20121211140541.23621.72514.stgit@srivatsabhat.in.ibm.com>
Date:	Tue, 11 Dec 2012 19:35:47 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org,
	vincent.guittot@...aro.org, tj@...nel.org, oleg@...hat.com
Cc:	sbw@....edu, amit.kucheria@...aro.org, rostedt@...dmis.org,
	rjw@...k.pl, srivatsa.bhat@...ux.vnet.ibm.com,
	wangyun@...ux.vnet.ibm.com, xiaoguangrong@...ux.vnet.ibm.com,
	nikunj@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH v4 9/9] 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_one_cpu() 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 by using get/put_online_cpus_atomic(), 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>
[ srivatsa.bhat@...ux.vnet.ibm.com: Refer to the new sync primitives for
  readers (in the changelog), and s/stop_cpus/stop_one_cpu ]
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
---

 kernel/cpu.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5a63296..3f9498e 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -484,7 +484,7 @@ 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);

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