linux-kernel - Re: [Question] load balance move tasks not suitable ?

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Message-ID: <20180730125606.GP2476@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Jul 2018 14:56:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "weiqi (C)" <weiqi4@...wei.com>
Cc:     linux-kernel@...r.kernel.org,
        Zhanghailiang <zhang.zhanghailiang@...wei.com>
Subject: Re: [Question] load balance move tasks not suitable ?

On Mon, Jul 30, 2018 at 08:08:55PM +0800, weiqi (C) wrote:
> Hi Peter,
> 
> After  Commits 5d6523ebd(sched: Fix load-balance wreckage)，

That's a _6_ year old patch... and I can barely remember last week.

> The jugement whether imbalance reached changed from twice to one-half.
> 
> from
> 
> (1) if((load * 2)> rem_load_move)
>         goto next;
> 
> to
> 
> (2) if((load / 2)> env-> load_move)
>         goto next;
> 
> I'm confused about this change.
> 
> "load*2" may be more appropriate, because if a task whose load more than
> env->imbalance is moved from high load cpu to low load cpu,
> 
> will make more imbalance.

It basically goes back to what it was before (see 367456c), in that
patch I (inadvertently) replaced (load.weight >> 1) with (load * 2)
instead of (load / 2).

I'm not entirely sure I can explain that logic at this time. Doing
software archeology on it might clarify where it came from.