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

[weiqi (C)]

in latest  linux kernel version,  load-balance's  detach_tasks() keeps  
"load/2"  behavior:

"""

        if (( load / 2 ) > env->imbalance)
                goto next;
"""

It  means  a task  "p",  which  load  is 1.5 times than "env->imbalance" 
can move from "src_rq" to "dst_rq".
after this move,  imbalance between  "src_rq"  and "dst_rq" may larger 
than  before.
but if use "load*2" here,  It means we can pick a task "p" , which load 
is half of "env->imbalance"ï and after moveï
"src_rq" and "dst_rq"  is balanced.

Am I right?


å 2018/7/30 20:56, Peter Zijlstra åé:
> On Mon, Jul 30, 2018 at 08:08:55PM +0800, weiqi (C) wrote:
> > Hi Peter,
> >
> > After  Commits 5d6523ebd(sched: Fix load-balance wreckage)ï
> 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.
> .