linux-kernel - Re: [PATCH] sched: don't check rq after newidle

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Message-ID: <20201215083238.GB3040@hirez.programming.kicks-ass.net>
Date:   Tue, 15 Dec 2020 09:32:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     chenxg1x@...il.com
Cc:     linux-kernel@...r.kernel.org, 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, heddchen@...cent.com,
        xiaoggchen@...cent.com
Subject: Re: [PATCH] sched: don't check rq after newidle_balance return
 positive

On Tue, Dec 15, 2020 at 02:48:50PM +0800, chenxg1x@...il.com wrote:
> From: Chen Xiaoguang <xiaoggchen@...cent.com>
> 
> In pick_next_task_fair, if CPU is going to idle newidle_balance
> is called first trying to pull some tasks.
> When newidle_balance returns positive which means it does
> pulls tasks or some tasks enqueued then there is no need to check
> sched_fair_runnable again.

No, I think it actually does need to, because while it counts the number
of tasks it pulled, it doesn't verify it still has them after it
re-acquires rq->lock. That is, someone could've come along and stolen
them right from under our noses.

> 
> Signed-off-by: He Chen <heddchen@...cent.com>
> Signed-off-by: Xiaoguang Chen <xiaoggchen@...cent.com>

This SoB chain is broken. The first SoB should be the author, but From
does not match.