linux-kernel - Re: [PATCH 01/18] sched: select_task_rq

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Date:	Tue, 11 Dec 2012 09:53:12 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Alex Shi <alex.shi@...el.com>
CC:	rob@...dley.net, mingo@...hat.com, peterz@...radead.org,
	gregkh@...uxfoundation.org, andre.przywara@....com, rjw@...k.pl,
	paul.gortmaker@...driver.com, akpm@...ux-foundation.org,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	pjt@...gle.com, vincent.guittot@...aro.org
Subject: Re: [PATCH 01/18] sched: select_task_rq_fair clean up

Hi Alex,

On 12/10/2012 01:52 PM, Alex Shi wrote:
> It is impossible to miss a task allowed cpu in a eligible group.

The one thing I am concerned with here is if there is a possibility of
the task changing its tsk_cpus_allowed() while this code is running.

i.e find_idlest_group() finds an idle group,then the tsk_cpus_allowed()
for the task changes,perhaps by the user himself,which might not include
the cpus in the idle group.After this find_idlest_cpu() is called.I mean
a race condition in short.Then we might not have an eligible cpu in that
group right?

> And since find_idlest_group only return a different group which
> excludes old cpu, it's also imporissible to find a new cpu same as old
> cpu.

This I agree with.

> Signed-off-by: Alex Shi <alex.shi@...el.com>
> ---
>  kernel/sched/fair.c |    5 -----
>  1 files changed, 0 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 59e072b..df99456 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3150,11 +3150,6 @@ select_task_rq_fair(struct task_struct *p, int sd_flag, int wake_flags)
>  		}
>  
>  		new_cpu = find_idlest_cpu(group, p, cpu);
> -		if (new_cpu == -1 || new_cpu == cpu) {
> -			/* Now try balancing at a lower domain level of cpu */
> -			sd = sd->child;
> -			continue;
> -		}
>  
>  		/* Now try balancing at a lower domain level of new_cpu */
>  		cpu = new_cpu;
> 
Regards
Preeti U Murthy

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