linux-kernel - [PATCH v5 43/45] cpu: No more __stop_machine() in _cpu

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Date:	Tue, 22 Jan 2013 13:14:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	tglx@...utronix.de, peterz@...radead.org, tj@...nel.org,
	oleg@...hat.com, paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	mingo@...nel.org, akpm@...ux-foundation.org, namhyung@...nel.org
Cc:	rostedt@...dmis.org, wangyun@...ux.vnet.ibm.com,
	xiaoguangrong@...ux.vnet.ibm.com, rjw@...k.pl, sbw@....edu,
	fweisbec@...il.com, linux@....linux.org.uk,
	nikunj@...ux.vnet.ibm.com, srivatsa.bhat@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org, netdev@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v5 43/45] 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.

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@...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); s/stop_cpus/stop_one_cpu and fix comment
  referring to stop_machine in the code]
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.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 1c84138..7a51fb6 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.
 	 */

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