linux-kernel - Re: [PATCH 3/3] sched: terminate newidle balancing once atleastone task has moved over

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Message-Id: <4860EEB3.BA47.005A.0@novell.com>
Date:	Tue, 24 Jun 2008 10:55:15 -0600
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	<mingo@...e.hu>, <rostedt@...dmis.org>, <tglx@...utronix.de>,
	"David Bahi" <DBahi@...ell.com>, <linux-kernel@...r.kernel.org>,
	<linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH 3/3] sched: terminate newidle balancing once
	atleastone task has moved over

>>> On Tue, Jun 24, 2008 at  9:31 AM, in message <1214314273.4351.34.camel@...ns>,
Peter Zijlstra <peterz@...radead.org> wrote: 
> On Tue, 2008-06-24 at 07:18 -0600, Gregory Haskins wrote:
>> >>> On Tue, Jun 24, 2008 at  6:13 AM, in message 
> <1214302406.4351.23.camel@...ns>,
>> Peter Zijlstra <peterz@...radead.org> wrote: 
>> > On Mon, 2008-06-23 at 17:04 -0600, Gregory Haskins wrote:
>> >> Inspired by Peter Zijlstra.
>> >> 
>> >> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>> >> ---
>> >> 
>> >>  kernel/sched.c |    4 ++++
>> >>  1 files changed, 4 insertions(+), 0 deletions(-)
>> >> 
>> >> diff --git a/kernel/sched.c b/kernel/sched.c
>> >> index 3efbbc5..c8e8520 100644
>> >> --- a/kernel/sched.c
>> >> +++ b/kernel/sched.c
>> >> @@ -2775,6 +2775,10 @@ static int move_tasks(struct rq *this_rq, int 
>> > this_cpu, struct rq *busiest,
>> >>  				max_load_move - total_load_moved,
>> >>  				sd, idle, all_pinned, &this_best_prio);
>> >>  		class = class->next;
>> >> +
>> >> +		if (idle == CPU_NEWLY_IDLE && this_rq->nr_running)
>> >> +			break;
>> >> +
>> >>  	} while (class && max_load_move > total_load_moved);
>> >>  
>> >>  	return total_load_moved > 0;
>> > 
>> > 
>> > right,.. uhm, except that you forgot all the other fixes and
>> > generalizations I had,..
>> 
>> Heh...well I intentionally simplified it, but perhaps that is out of 
> ignorance.  I did say "inspired by" ;)
>> 
>> > 
>> > The LB_START/LB_COMPLETE stuff is needed to fix CFS load balancing. It
>> > now always iterates the first sysctl_sched_nr_migrate tasks, and if it
>> > doesn't find any there, just gives up - which isn't too big of a problem
>> > with it set to 32, but if you drop it to 2/4 stuff starts valing apart.
>> > 
>> > And the break I had here, only checks classes above and equal to the
>> > current class.
>> > 
>> > This again is needed when you have more classes.
>> 
>> Im not sure I understand/agree here (unless you plan on having a class below 
> sched_idle()??)
>> 
>> The fact that we are going NEWLYIDLE to me implies that all the other 
> classes are
>> "above or equal".  And rq->nr_running approximates all the per-class vtable 
> work
>> that you had done to probe the higher classes.  We currently only hit this 
> code when
>> rq->nr_running == 0, so rq->nr_running !=0 seems like a logical termination
>> condition.
>> 
>> I guess what I am not clear on is: "when would we be NEWLYIDLE in a higher 
> class,
>> yet have tasks populated in lower classes such at nr_running is non-zero".
>> Additionally, even if we have that condition (e.g. with something like the 
> EDF work you
>> are doing, perhaps?), shouldn't we patch the advanced form of this logic 
> when the rest
>> of the code goes in?  For now, this seems like the most straight forward way 
> to
>> accomplish the goal.  But I could be missing something ;)
> 
> The thing I'm worried about - but it might be unfounded and is certainly
> so now - is that suppose we have:
> 
>   EDF
>   FIFO/RR
>   SOFTRT
>   OTHER
>   IDLE
> 
> and we've just done FIFO/RR (which is a nop) and and some interrupt woke
> an OTHER task while we dropped for lockbreak.
> 
> At this point your logic would bail out and start running the OTHER
> task, even though we might have found a SOFTRQ task to run had we
> bothered to look.
> 

Ok, now I think I understand your concern.  But I think you may be worrying about
this at the wrong level.  I would think we should be doing something similar to the
post-balance patch I submitted a while back.  It basically iterated through each class,
giving each an opportunity to pull tasks over in its own way.  The difference there
was that I was doing it post-schedule to deal with that locking issue.  We could
take the same idea and do it where we pre_schedule() today.

I think the f_b_g() et. al. is really SCHED_OTHER specific, and probably always will be.
Lets just formalize that.  Perhaps we should move all the LB code to sched_fair and set
something like what I proposed up.  Thoughts?

-Greg



> 
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