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

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Message-ID: <Pine.LNX.4.58.0806232104460.8540@gandalf.stny.rr.com>
Date:	Mon, 23 Jun 2008 21:07:53 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Gregory Haskins <ghaskins@...ell.com>, mingo@...e.hu,
	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	peterz@...radead.org, linux-rt-users@...r.kernel.org,
	dbahi@...ell.com
Subject: Re: [PATCH 3/3] sched: terminate newidle balancing once at least
 one task has moved over


On Tue, 24 Jun 2008, Nick Piggin wrote:

>
> On Tuesday 24 June 2008 09:04, Gregory Haskins wrote:
> > Inspired by Peter Zijlstra.
>
> Is this really getting tested well? Because at least for SCHED_OTHER
> tasks, the newidle balancer is still supposed to be relatively
> conservative and not over balance too much. By the time you have
> done all this calculation and reached here, it will be a loss to only
> move one task if you could have moved two and halved your newidle
> balance rate...

We've been finding a lot of our high latencies have been coming from the
balancing code. And the newidle balance is a large offender. I don't think
it's much wasted work for what you want. Even if we wasted the work done,
it was during "idle" time. But now we have a task to run, why not run it
now. Especially if that task is an RT task and doesn't like to wait.

The newidle balance should really just get a task to run, the balancing
code should be done at a later time. Ideally when no RT tasks need to run.

-- Steve


>
> > 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;
> >
> > --
>
--
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