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

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Date:   Mon, 24 Feb 2020 00:16:56 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     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 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.

[...]