linux-kernel - Re: [PATCH] sched: Remove broken check for skip clock update

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Message-ID: <1357609797.5190.17.camel@gandalf.local.home>
Date:	Mon, 07 Jan 2013 20:49:57 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] sched: Remove broken check for skip clock update

On Tue, 2013-01-08 at 02:07 +0100, Frederic Weisbecker wrote:
> rq->skip_clock_update shouldn't be negative. Thus the check
> in put_prev_task() is useless.
> 
> It was probably intended to do the following check:
> 
> 	if (prev->on_rq && !rq->skip_clock_update)
> 
> We only want to update the clock if the current task is
> not voluntarily sleeping: otherwise deactivate_task()
> already did the rq clock update in schedule(). But we want
> to ignore that update if a ttwu did it for us, in which case
> rq->skip_clock_update is 1.
> 
> But update_rq_clock() already takes care of that so we
> can just remove the broken condition.

kernel/sched/rt.c:

                                /*
                                 * Force a clock update if the CPU was idle,
                                 * lest wakeup -> unthrottle time accumulate.
                                 */
                                if (rt_rq->rt_nr_running && rq->curr == rq->idle)
                                        rq->skip_clock_update = -1;


-- Steve

> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/sched/core.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 15ba35e..8dfc461 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2886,7 +2886,7 @@ static inline void schedule_debug(struct task_struct *prev)
>  
>  static void put_prev_task(struct rq *rq, struct task_struct *prev)
>  {
> -	if (prev->on_rq || rq->skip_clock_update < 0)
> +	if (prev->on_rq)
>  		update_rq_clock(rq);
>  	prev->sched_class->put_prev_task(rq, prev);
>  }


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