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

From: Morten Rasmussen <morten.rasmussen@arm.com>
To: Lei Wen <adrian.wenl@gmail.com>
Cc: Alex Shi <alex.shi@intel.com>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"bp@alien8.de" <bp@alien8.de>, "pjt@google.com" <pjt@google.com>,
	"namhyung@kernel.org" <namhyung@kernel.org>,
	"efault@gmx.de" <efault@gmx.de>,
	"vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
	"preeti@linux.vnet.ibm.com" <preeti@linux.vnet.ibm.com>,
	"viresh.kumar@linaro.org" <viresh.kumar@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"mgorman@suse.de" <mgorman@suse.de>,
	"riel@redhat.com" <riel@redhat.com>,
	"wangyun@linux.vnet.ibm.com" <wangyun@linux.vnet.ibm.com>,
	Jason Low <jason.low2@hp.com>,
	Changlong Xie <changlongx.xie@intel.com>,
	"sgruszka@redhat.com" <sgruszka@redhat.com>,
	"fweisbec@gmail.com" <fweisbec@gmail.com>
Subject: Re: [patch v8 3/9] sched: set initial value of runnable avg for new forked task
Date: Thu, 20 Jun 2013 11:23:34 +0100	[thread overview]
Message-ID: <20130620102334.GI5460@e103034-lin> (raw)
In-Reply-To: <CALZhoSTxKUD2oE7YUFvwASKqnpJBgPP6mnJG3KhFUre5OqYBFQ@mail.gmail.com>

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@intel.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
> 