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

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Date:	Thu, 20 Jun 2013 11:23:34 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Lei Wen <adrian.wenl@...il.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

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.

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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