linux-kernel - Re: [PATCH v2] sched: Accelerate "pick_next

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Message-ID: <CAJn8CcHhaSA5kF9KVjMorzt_2NM-dTPc5mTr3-A9GvR54VRo=A@mail.gmail.com>
Date:	Tue, 17 Jan 2012 10:58:15 +0800
From:	Xiaotian Feng <xtfeng@...il.com>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: Accelerate "pick_next_entity" under special condition

On Tue, Jan 17, 2012 at 10:36 AM, Michael Wang
<wangyun@...ux.vnet.ibm.com> wrote:
> From: wangyun <wangyun@...ux.vnet.ibm.com>
>
> In original code, we get the next entity in this way:
>
>        if(condition1)
>                result=value1;
>        if(condition2)
>                result=value2;
>        if(condition3)
>                result=value3;
>        return result;
>
> So if condition3 is true, we will get value3, but still
> need to check condition1 and condition2, this will waste
> our time.
>
> This patch will change the way like:
>
>        if(condition3) {
>                result=value3;
>                goto out;
>        }
>        if(condition2) {
>                result=value2;
>                goto out;
>        }
>        if(condition1) {
>                result=value1;
>                goto out;
>        }
>
>        out:
>        return result;
>
> So we can avoid check condition2 and condition1 when
> condition3 is true now.

Then what if condition 1 is true now?

>
> v2:
>        1. do not use ugly macro any more.
>        2. add more description.
>
> Signed-off-by: Michael Wang <wangyun@...ux.vnet.ibm.com>
> ---
>  kernel/sched/fair.c |   26 +++++++++++++++-----------
>  1 files changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 84adb2d..e8a72b2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1308,29 +1308,33 @@ static struct sched_entity
> *pick_next_entity(struct cfs_rq *cfs_rq)
>        struct sched_entity *left = se;
>
>        /*
> -        * Avoid running the skip buddy, if running something else can
> -        * be done without getting too unfair.
> +        * Someone really wants this to run. If it's not unfair, run it.
>         */
> -       if (cfs_rq->skip == se) {
> -               struct sched_entity *second = __pick_next_entity(se);
> -               if (second && wakeup_preempt_entity(second, left) < 1)
> -                       se = second;
> +       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
> +               se = cfs_rq->next;
> +               goto out;
>        }
>
>        /*
>         * Prefer last buddy, try to return the CPU to a preempted task.
>         */
> -       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
> +       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
>                se = cfs_rq->last;
> +               goto out;
> +       }
>
>        /*
> -        * Someone really wants this to run. If it's not unfair, run it.
> +        * Avoid running the skip buddy, if running something else can
> +        * be done without getting too unfair.
>         */
> -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
> -               se = cfs_rq->next;
> +       if (cfs_rq->skip == se) {
> +               struct sched_entity *second = __pick_next_entity(se);
> +               if (second && wakeup_preempt_entity(second, left) < 1)
> +                       se = second;
> +       }
>
> +out:
>        clear_buddies(cfs_rq, se);
> -
>        return se;
>  }
>
> --
> 1.7.4.1
>
> --
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