linux-kernel - Re: [PATCH] sched/rt: Make update_curr

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Date:   Mon, 5 Feb 2018 11:57:08 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Wen Yang <wen.yang99@....com.cn>
Cc:     mingo@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, jiang.biao2@....com.cn,
        zhong.weidong@....com.cn
Subject: Re: [PATCH] sched/rt: Make update_curr_rt() more accurate


* Wen Yang <wen.yang99@....com.cn> wrote:

> rq->clock_task may be updated between the two calls of 
> rq_clock_task() in update_curr_rt(). Calling rq_clock_task() only
> once makes it more accurate and efficient, taking update_curr() as
> reference.
> 
> Signed-off-by: Wen Yang <wen.yang99@....com.cn>
> Reviewed-by: Jiang Biao <jiang.biao2@....com.cn>
> ---
>  kernel/sched/rt.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4056c19..d6d2a65 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -950,12 +950,13 @@ static void update_curr_rt(struct rq *rq)
>  {
>  	struct task_struct *curr = rq->curr;
>  	struct sched_rt_entity *rt_se = &curr->rt;
> +	u64 now = rq_clock_task(rq);
>  	u64 delta_exec;
>  
>  	if (curr->sched_class != &rt_sched_class)
>  		return;
>  
> -	delta_exec = rq_clock_task(rq) - curr->se.exec_start;
> +	delta_exec = now - curr->se.exec_start;

Small nit: shouldn't we calculate 'now' after the return?

OTOH that 'return' should only be triggered statistically AFAICS, i.e. very 
rarely.

Thanks,

	Ingo