linux-kernel - Re: [patch v3 6/8] sched: consider runnable load average in move

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Message-ID: <CAKfTPtDbiv2ACgem7k7YxcxgwGA4-JY1LcAg0B_XOVaOeE8aeQ@mail.gmail.com>
Date:	Tue, 9 Apr 2013 10:58:36 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, Paul Turner <pjt@...gle.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	gregkh@...uxfoundation.org,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>, rafael.j.wysocki@...el.com,
	jkosina@...e.cz, clark.williams@...il.com,
	"tony.luck@...el.com" <tony.luck@...el.com>, keescook@...omium.org,
	mgorman@...e.de, riel@...hat.com
Subject: Re: [patch v3 6/8] sched: consider runnable load average in move_tasks

On 9 April 2013 10:05, Alex Shi <alex.shi@...el.com> wrote:
> On 04/09/2013 03:08 PM, Vincent Guittot wrote:
>> On 2 April 2013 05:23, Alex Shi <alex.shi@...el.com> wrote:
>>> Except using runnable load average in background, move_tasks is also
>>> the key functions in load balance. We need consider the runnable load
>>> average in it in order to the apple to apple load comparison.
>>>
>>> Signed-off-by: Alex Shi <alex.shi@...el.com>
>>> ---
>>>  kernel/sched/fair.c | 11 ++++++++++-
>>>  1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 1f9026e..bf4e0d4 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -3966,6 +3966,15 @@ static unsigned long task_h_load(struct task_struct *p);
>>>
>>>  static const unsigned int sched_nr_migrate_break = 32;
>>>
>>> +static unsigned long task_h_load_avg(struct task_struct *p)
>>> +{
>>> +       u32 period = p->se.avg.runnable_avg_period;
>>> +       if (!period)
>>> +               return 0;
>>> +
>>> +       return task_h_load(p) * p->se.avg.runnable_avg_sum / period;
>>
>> How do you ensure that runnable_avg_period and runnable_avg_sum are
>> coherent ? an update of the statistic can occur in the middle of your
>> sequence.
>
> Thanks for your question, Vincent!
> the runnable_avg_period and runnable_avg_sum, only updated in
> __update_entity_runnable_avg().
> Yes, I didn't see some locks to ensure the coherent of them. but they
> are updated closely, and it is not big deal even a little incorrect to
> their value. These data are collected periodically, don't need very very
> precise at every time.
> Am I right? :)

The problem mainly appears during starting phase (the 1st 345ms) when
runnable_avg_period has not reached the max value yet so you can have
avg.runnable_avg_sum greater than avg.runnable_avg_period. In a worst
case, runnable_avg_sum could be twice runnable_avg_period

Vincent

>
> --
> Thanks Alex
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