linux-kernel - Re: [patch v8 3/9] sched: set initial value of runnable avg for new forked task

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Date:	Fri, 21 Jun 2013 10:57:27 +0800
From:	Lei Wen <adrian.wenl@...il.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Alex Shi <alex.shi@...el.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"bp@...en8.de" <bp@...en8.de>, "pjt@...gle.com" <pjt@...gle.com>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	"efault@....de" <efault@....de>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"viresh.kumar@...aro.org" <viresh.kumar@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mgorman@...e.de" <mgorman@...e.de>,
	"riel@...hat.com" <riel@...hat.com>,
	"wangyun@...ux.vnet.ibm.com" <wangyun@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Changlong Xie <changlongx.xie@...el.com>,
	"sgruszka@...hat.com" <sgruszka@...hat.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>
Subject: Re: [patch v8 3/9] sched: set initial value of runnable avg for new
 forked task

Morten,

On Thu, Jun 20, 2013 at 6:23 PM, Morten Rasmussen
<morten.rasmussen@....com> wrote:
> On Sat, Jun 15, 2013 at 01:09:12PM +0100, Lei Wen wrote:
>> On Fri, Jun 14, 2013 at 9:59 PM, Alex Shi <alex.shi@...el.com> wrote:
>> > On 06/14/2013 06:02 PM, Lei Wen wrote:
>> >>> >         enqueue_entity
>> >>> >             enqueue_entity_load_avg
>> >>> >
>> >>> > and make forking balancing imbalance since incorrect load_avg_contrib.
>> >>> >
>> >>> > Further more, Morten Rasmussen notice some tasks were not launched at
>> >>> > once after created. So Paul and Peter suggest giving a start value for
>> >>> > new task runnable avg time same as sched_slice().
>> >> I am confused at this comment, how set slice to runnable avg would change
>> >> the behavior of "some tasks were not launched at once after created"?
>> >
>> > I also don't know the details on Morten's machine. but just guess, there
>> > are much tasks on in the run queue. the minimum load avg make the new
>> > task wait its time...
>>
>> Is there some possibility that since task structure is allocated without being
>> set to 0, and it cause the imbalance between runqueues. Then the new forked
>> is migrated to other cpus, so that it cause its execution being delayed?
>>
>> It is better for Morten to give us more details here. :)
>>
>
> I think Peter's reply pretty much covers it. The problem is when a task
> is not running (other task has lower vruntime or blocked for other
> reasons) shortly after the task was created. The runnable_avg_period is
> very small, so the load_contrib is very sensitive.
>
> Say if a task runs for 1 ms then is blocked for 1 ms and then runs
> again, the load_contrib will go from 100% to 50% instantly and then ramp
> back up again. So the task load may be quite different from the true
> load of the task depending on when you calculate the load_contrib.
>
> Preloading runnable_avg_period should make the load_contrib a little
> less sensitive to this behaviour.

Thanks for detailed explanation!
Now I could understand the preloading value prevent entity's load change quickly
at its beginning.
But I cannot see why this could explain "some tasks were not launched at
once after created"

Or should I understand the previous words as:
"If without preloading value, the new created task's contributed load may vary
too quickly at its beginning. Like if a task runs for 1 ms then is
blocked for 1 ms
and then runs again, the load_contrib will go from 100% to 50% instantly."


Thanks,
Lei

>
> Morten
>
>> Thanks,
>> Lei
>>
>> >>
>> >> IMHO, I could only tell that for the new forked task, it could be run if current
>> >> task already be set as need_resched, and preempt_schedule or
>> >> preempt_schedule_irq
>> >> is called.
>> >>
>> >> Since the set slice to avg behavior would not affect this task's vruntime,
>> >> and hence cannot make current running task be need_sched, if
>> >> previously it cannot.
>> >>
>> >> Could you help correct if I am wrong at somewhere? ....
>> >>
>> >> Thanks,
>> >
>> >
>> > --
>> > Thanks
>> >     Alex
>>
>
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