diff mbox

[RFC,v2,10/10] cpu: No more __stop_machine() in _cpu_down()

Message ID 20121205184456.3750.35005.stgit@srivatsabhat.in.ibm.com (mailing list archive)
State RFC, archived
Headers show

Commit Message

Srivatsa S. Bhat Dec. 5, 2012, 6:45 p.m. UTC 
From: Paul E. McKenney <paul.mckenney@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_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 by using get/put_online_cpus_atomic_light(), or
get/put_online_cpus_atomic_full(), but in the meantime, this commit is one
way to help locate them.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ Srivatsa: Refer to the new sync primitives for readers, in the changelog ]
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

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


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Comments

Oleg Nesterov Dec. 5, 2012, 7:08 p.m. UTC | #1 
On 12/06, Srivatsa S. Bhat wrote:
>
> @@ -418,7 +418,7 @@ 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);

stop_one_cpu(cpu) ?

Oleg.

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Srivatsa S. Bhat Dec. 5, 2012, 7:12 p.m. UTC | #2 
On 12/06/2012 12:38 AM, Oleg Nesterov wrote:
> On 12/06, Srivatsa S. Bhat wrote:
>>
>> @@ -418,7 +418,7 @@ 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);
> 
> stop_one_cpu(cpu) ?
> 

Even I was thinking of using that. Paul, any particular reason you chose
stop_cpus() over stop_one_cpu() in [1]?

[1]. https://lkml.org/lkml/2012/10/30/359

Regards,
Srivatsa S. Bhat

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diff mbox

Patch

diff --git a/kernel/cpu.c b/kernel/cpu.c
index c71c723..00a1edc 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -418,7 +418,7 @@  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);