linux-kernel - Re: [PATCH 1/2] sched/rt: Use 'back' only if CONFIG_RT_GROUP

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Date: Tue, 4 Jun 2024 09:22:03 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Yajun Deng <yajun.deng@...ux.dev>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
 vincent.guittot@...aro.org, dietmar.eggemann@....com, bsegall@...gle.com,
 mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/rt: Use 'back' only if CONFIG_RT_GROUP_SCHED
 is enabled

On Mon,  3 Jun 2024 20:50:01 +0800
Yajun Deng <yajun.deng@...ux.dev> wrote:

> The 'back' member in struct sched_rt_entity only related to
> CONFIG_RT_GROUP_SCHED, so there is no need to init it in dequeue_rt_stack.
> 
> Init the 'back' member in init_tg_rt_entry and use it only if
> CONFIG_RT_GROUP_SCHED is enabled.
> 
> Signed-off-by: Yajun Deng <yajun.deng@...ux.dev>
> ---
>  kernel/sched/rt.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 63e49c8ffc4d..44bb9087cfe5 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -232,8 +232,10 @@ void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
>  
>  	if (!parent)
>  		rt_se->rt_rq = &rq->rt;
> -	else
> +	else {
>  		rt_se->rt_rq = parent->my_q;
> +		parent->back = rt_se;
> +	}

If one block of an if/else requires brackets, then both do.


>  
>  	rt_se->my_q = rt_rq;
>  	rt_se->parent = parent;
> @@ -1428,16 +1430,19 @@ static void dequeue_rt_stack(struct sched_rt_entity *rt_se, unsigned int flags)
>  	struct sched_rt_entity *back = NULL;
>  	unsigned int rt_nr_running;
>  
> -	for_each_sched_rt_entity(rt_se) {
> -		rt_se->back = back;
> +	for_each_sched_rt_entity(rt_se)
>  		back = rt_se;
> -	}
>  
>  	rt_nr_running = rt_rq_of_se(back)->rt_nr_running;
>  
> -	for (rt_se = back; rt_se; rt_se = rt_se->back) {
> +	for (rt_se = back; rt_se; ) {
>  		if (on_rt_rq(rt_se))
>  			__dequeue_rt_entity(rt_se, flags);
> +#ifdef CONFIG_RT_GROUP_SCHED
> +		rt_se = rt_se->back;
> +#else
> +		rt_se = NULL;
> +#endif

Perhaps make a helper function to get rid of the #ifdef in the code here.

	for (rt_se = back, rt_se; rt_se = rt_se_back(rt_se)) {

Where rt_se_back() above can be defined in the #ifdef blocks for
CONFIG_RT_GROUP_SCHED as either:

static struct sched_rt_entity *rt_se_back(struct sched_rt_entity *se)
{
	return se->back;
}

Or

static inline struct sched_rt_entity *rt_se_back(struct sched_rt_entity *se)
{
	return NULL;
}

-- Steve


>  	}
>  
>  	dequeue_top_rt_rq(rt_rq_of_se(back), rt_nr_running);