linux-kernel - Re: [PATCH 1/2] sched: push rt tasks only if newly activated tasks have been added

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Date:	Wed, 23 Apr 2008 12:23:25 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Gregory Haskins" <ghaskins@...ell.com>
Cc:	"Steven Rostedt" <rostedt@...dmis.org>, mingo@...e.hu,
	chinang.ma@...el.com, suresh.b.siddha@...el.com,
	arjan@...ux.intel.com, willy@...ux.intel.com,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH 1/2] sched: push rt tasks only if newly activated tasks have been added

2008/4/23 Gregory Haskins <ghaskins@...ell.com>:
> [ ... ]
>
>  I think we can simplify this further.  We really only need to push here if we are not going to reschedule anytime soon (probably white-space damaged):
>
>
>  --- a/kernel/sched_rt.c
>
> +++ b/kernel/sched_rt.c
>  @@ -1058,11 +1058,14 @@ static void post_schedule_rt(struct rq *rq)
>         }
>   }
>
>  -
>  +/*
>  + * If we are not running and we are not going to reschedule soon, we should
>  + * try to push tasks away now
>  + */
>
>  static void task_wake_up_rt(struct rq *rq, struct task_struct *p)
>   {
>         if (!task_running(rq, p) &&
>  -           (p->prio >= rq->rt.highest_prio) &&
>  +           !test_tsk_thread_flag(rq->curr, TIF_NEED_RESCHED) &&
>             rq->rt.overloaded)
>                 push_rt_tasks(rq);
>   }


It's somewhat suboptimal as it doesn't guarantee that 'p' gets control next.

e.g. 2 tasks (T0 and T1) have been woken up before an actual
re-schedule takes place. Even if T1 is of lower prio than T0,
task_wake_up_rt() will see the NEED_RESCHED flag and bail out while it
would make sense at this moment to push T1 off this cpu.


p.s. hope you are better today. get well! :-)


-- 
Best regards,
Dmitry Adamushko
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