linux-kernel - Re: [PATCH 04/16] sched/fair: Optimize find_idlest

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Date:	Tue, 7 Jun 2016 15:25:21 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/16] sched/fair: Optimize find_idlest_cpu() when there
 is no choice

On Wed, Jun 01, 2016 at 09:59:20PM +0200, Peter Zijlstra wrote:
> On Mon, May 23, 2016 at 11:58:46AM +0100, Morten Rasmussen wrote:
> > In the current find_idlest_group()/find_idlest_cpu() search we end up
> > calling find_idlest_cpu() in a sched_group containing only one cpu in
> > the end. Checking idle-states becomes pointless when there is no
> > alternative, so bail out instead.
> > 
> > cc: Ingo Molnar <mingo@...hat.com>
> > cc: Peter Zijlstra <peterz@...radead.org>
> > 
> > Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> > ---
> >  kernel/sched/fair.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 0fe3020..564215d 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5155,6 +5155,11 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
> >  	int shallowest_idle_cpu = -1;
> >  	int i;
> >  
> > +	/* Check if we have any choice */
> > +	if (group->group_weight == 1) {
> > +		return cpumask_first(sched_group_cpus(group));
> > +	}
> 
> superfluous brackets, also isn't @this_cpu supposed to be part of the
> sched_group_cpu(group) mask?

I'll kill the brackets.

IIUC, @this_cpu is never part of sched_group_cpus(group).
find_idlest_group() never returns the local group, so if we manage to
get to find_idlest_cpu(), the group would not contain @this_cpu.