linux-kernel - 答复: [Resend][PATCH] sched/fair: micro-optimize pick_next

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Message-ID: <ccbb34c46fe74d11a93c8514f0ba9936@baidu.com>
Date:   Fri, 3 Sep 2021 03:59:08 +0000
From:   "Li,Rongqing" <lirongqing@...du.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "mingo@...hat.com" <mingo@...hat.com>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
        "dietmar.eggemann@....com" <dietmar.eggemann@....com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "bristot@...hat.com" <bristot@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: 答复: [Resend][PATCH] sched/fair: micro-optimize pick_next_entity()



> -----邮件原件-----
> 发件人: Peter Zijlstra <peterz@...radead.org>
> 发送时间: 2021年8月31日 22:10
> 收件人: Li,Rongqing <lirongqing@...du.com>
> 抄送: mingo@...hat.com; juri.lelli@...hat.com; vincent.guittot@...aro.org;
> dietmar.eggemann@....com; rostedt@...dmis.org; bsegall@...gle.com;
> mgorman@...e.de; bristot@...hat.com; linux-kernel@...r.kernel.org
> 主题: Re: [Resend][PATCH] sched/fair: micro-optimize pick_next_entity()
> 
> On Wed, Aug 25, 2021 at 02:27:49PM +0800, Li RongQing wrote:
> > Only check the skip buddy when next buddy and last buddy are not
> > picked up, this can save the cycles of checking the skip buddy and
> > computation of the second buddy, when next and last buddy will be
> > picked up for example, yield_to_task_fair() set both next and skip
> > buddy
> 
> Is that actually measurable?
> 
> But looking at it, should we not, instead, move the whole ->skip thing to the
> bottom, so we unconditionally check it vs the result of
> ->next/->last ?
> 
> Imagine ->next == ->skip, then we want to avoid running it and not have
> ->next win.

skip means this task is running on cpu, next is the blocked task and will be running.

I donot know how they can be same.

-Li