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

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Message-ID: <50C6C462.6030808@intel.com>
Date:	Tue, 11 Dec 2012 13:28:02 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Preeti U Murthy <preeti@...ux.vnet.ibm.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

On 12/11/2012 12:23 PM, Preeti U Murthy wrote:
> 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?

your worry make sense, but the code handle the situation, in
select_task_rq(), it will check the cpu allowed again. if the answer is
no, it will fallback to old cpu.
> 
>> 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
> 


-- 
Thanks
    Alex
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