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

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Message-Id: <20190708140751.GA10675@linux.vnet.ibm.com>
Date:   Mon, 8 Jul 2019 19:37:51 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
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()

* Peter Zijlstra <peterz@...radead.org> [2019-07-08 12:23:12]:

> 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).
> 

Yes, it definitely makes code readable. I was only commenting on the
changelog/subject which says avoids a function call which I think it
doesn't.

-- 
Thanks and Regards
Srikar Dronamraju