linux-kernel - Re: [PATCH v2 6/6] sched/rt: Remove unnecessary assignment in inc/dec_rt

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Message-ID: <20200224123123.gbox3tcqcist7bbg@e107158-lin.cambridge.arm.com>
Date:   Mon, 24 Feb 2020 12:31:23 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/6] sched/rt: Remove unnecessary assignment in
 inc/dec_rt_migration

On 02/24/20 00:16, Dietmar Eggemann wrote:
> On 23.02.20 19:40, Qais Yousef wrote:
> > The statement
> > 
> > 	rt_rq = &rq_of_rt_rq(rt_rq)->rt
> > 
> > Was just dereferencing rt_rq to get a pointer to itself. Which is a NOP.
> > Remove it.
> > 
> > Signed-off-by: Qais Yousef <qais.yousef@....com>
> > ---
> >  kernel/sched/rt.c | 2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> > index b35e49cdafcc..520e84993fe7 100644
> > --- a/kernel/sched/rt.c
> > +++ b/kernel/sched/rt.c
> > @@ -343,7 +343,6 @@ static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> >  		return;
> >  
> >  	p = rt_task_of(rt_se);
> > -	rt_rq = &rq_of_rt_rq(rt_rq)->rt;
> 
> IMHO, this is here to get the root rt_rq from any rt_rq (task_groups).
> Looks like that e.g rt_nr_total is only maintained on root rt_rq's.
> 
> Similar to CFS' &rq_of(cfs_rq)->cfs (cfs_rq_util_change()) to get root
> cfs_rq.
> 
> Not sure where CONFIG_RT_GROUP_SCHED=y is used but it's part of the rt
> class implementation.

Ah I see. That was obvious.. How about the below comment?

This code is executed only if rt_entity_is_task(), I don't think this grantees
that the rt_rq isn't for a group?

I need to go and unravel the layers maybe.

Thanks!

--
Qais Yousef

-->8--

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index b35e49cdafcc..f929867215c4 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -343,6 +343,8 @@ static void inc_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
                return;

        p = rt_task_of(rt_se);
+
+       /* get the root rt_rq if this is the rt_rq of a group */
        rt_rq = &rq_of_rt_rq(rt_rq)->rt;

        rt_rq->rt_nr_total++;
@@ -368,6 +370,8 @@ static void dec_rt_migration(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
                return;

        p = rt_task_of(rt_se);
+
+       /* get the root rt_rq if this is the rt_rq of a group */
        rt_rq = &rq_of_rt_rq(rt_rq)->rt;

        rt_rq->rt_nr_total--;