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

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Date:	Tue, 24 Jun 2008 15:31:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
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 at
	leastone task has moved over

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.



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