[PATCH] sched: fix off-by-one bug in balance_tasks()

[PATCH] sched: fix off-by-one bug in balance_tasks()

[Miao Xie]

If the load that need be moved equals the half weight of a task, I think
it is unnecessary to move this task. Or this task will be moved back and
forth.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
---
 kernel/sched.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 5724508..44926c8 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3085,7 +3085,7 @@ next:
 	if (!p || loops++ > sysctl_sched_nr_migrate)
 		goto out;
 
-	if ((p->se.load.weight >> 1) > rem_load_move ||
+	if ((p->se.load.weight >> 1) >= rem_load_move ||
 	    !can_migrate_task(p, busiest, this_cpu, sd, idle, &pinned)) {
 		p = iterator->next(iterator->arg);
 		goto next;
-- 
1.6.0.3

Re: [PATCH] sched: fix off-by-one bug in balance_tasks()

[Peter Zijlstra]

On Wed, 2009-04-15 at 10:49 +0800, Miao Xie wrote:
> If the load that need be moved equals the half weight of a task, I think
> it is unnecessary to move this task. Or this task will be moved back and
> forth.

That's actually desirable. Consider the 3 tasks on 2 cpus statically
infeasible scenario. There you'd want the tasks to bounce around a bit
in order to provide fairness.

> Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
> ---
>  kernel/sched.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 5724508..44926c8 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -3085,7 +3085,7 @@ next:
>  	if (!p || loops++ > sysctl_sched_nr_migrate)
>  		goto out;
>  
> -	if ((p->se.load.weight >> 1) > rem_load_move ||
> +	if ((p->se.load.weight >> 1) >= rem_load_move ||
>  	    !can_migrate_task(p, busiest, this_cpu, sd, idle, &pinned)) {
>  		p = iterator->next(iterator->arg);
>  		goto next;

Re: [PATCH] sched: fix off-by-one bug in balance_tasks()

[Miao Xie]

on 2009-4-15 19:41 Peter Zijlstra wrote:
> On Wed, 2009-04-15 at 10:49 +0800, Miao Xie wrote:
>> If the load that need be moved equals the half weight of a task, I think
>> it is unnecessary to move this task. Or this task will be moved back and
>> forth.
> 
> That's actually desirable. Consider the 3 tasks on 2 cpus statically
> infeasible scenario. There you'd want the tasks to bounce around a bit
> in order to provide fairness.

I see. Thanks for your explanation.

Miao Xie
> 
>> Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
>> ---
>>  kernel/sched.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/kernel/sched.c b/kernel/sched.c
>> index 5724508..44926c8 100644
>> --- a/kernel/sched.c
>> +++ b/kernel/sched.c
>> @@ -3085,7 +3085,7 @@ next:
>>  	if (!p || loops++ > sysctl_sched_nr_migrate)
>>  		goto out;
>>  
>> -	if ((p->se.load.weight >> 1) > rem_load_move ||
>> +	if ((p->se.load.weight >> 1) >= rem_load_move ||
>>  	    !can_migrate_task(p, busiest, this_cpu, sd, idle, &pinned)) {
>>  		p = iterator->next(iterator->arg);
>>  		goto next;
> 
> 
> 
>