linux-kernel - Re: [PATCH] sched/topology: One function call less in build_group_from_child_sched

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Message-ID: <20190708102312.GF3402@hirez.programming.kicks-ass.net>
Date:   Mon, 8 Jul 2019 12:23:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Markus Elfring <Markus.Elfring@....de>,
        kernel-janitors@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/topology: One function call less in
 build_group_from_child_sched_domain()

On Sat, Jul 06, 2019 at 10:52:23PM +0530, Srikar Dronamraju wrote:
> * Markus Elfring <Markus.Elfring@....de> [2019-07-06 16:05:17]:
> 
> > From: Markus Elfring <elfring@...rs.sourceforge.net>
> > Date: Sat, 6 Jul 2019 16:00:13 +0200
> > 
> > Avoid an extra function call by using a ternary operator instead of
> > a conditional statement.
> > 
> > This issue was detected by using the Coccinelle software.
> > 
> > Signed-off-by: Markus Elfring <elfring@...rs.sourceforge.net>
> > ---
> >  kernel/sched/topology.c | 6 +-----
> >  1 file changed, 1 insertion(+), 5 deletions(-)
> > 
> > diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> > index f751ce0b783e..6190eb52c30a 100644
> > --- a/kernel/sched/topology.c
> > +++ b/kernel/sched/topology.c
> > @@ -886,11 +886,7 @@ build_group_from_child_sched_domain(struct sched_domain *sd, int cpu)
> >  		return NULL;
> > 
> >  	sg_span = sched_group_span(sg);
> > -	if (sd->child)
> > -		cpumask_copy(sg_span, sched_domain_span(sd->child));
> > -	else
> > -		cpumask_copy(sg_span, sched_domain_span(sd));
> > -
> > +	cpumask_copy(sg_span, sched_domain_span(sd->child ? sd->child : sd));
> 
> At runtime, Are we avoiding a function call?
> However I think we are avoiding a branch instead of a conditional, which may
> be beneficial.

It all depends on what the compiler does; also this is super slow path
stuff and the patch makes code less readable (IMO).