linux-kernel - Re: [PATCH v2] sched: Fast idling of CPU when system is partially loaded

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Date:	Mon, 23 Jun 2014 11:59:40 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Jason Low <jason.low2@...com>,
	Davidlohr Bueson <davidlohr@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: Fast idling of CPU when system is partially
 loaded

On Mon, 2014-06-23 at 20:50 +0200, Peter Zijlstra wrote:
> On Mon, Jun 23, 2014 at 09:40:45AM -0700, Tim Chen wrote:
> 
> > > @@ -5886,7 +5886,7 @@ static inline void update_sg_lb_stats(st
> > >  
> > >  		sgs->group_load += load;
> > >  		sgs->sum_nr_running += rq->nr_running;
> > > -		if (overload && rq->nr_running > 1)
> > > +		if (rq->nr_running > 1)
> > >  			*overload = true;
> > >  #ifdef CONFIG_NUMA_BALANCING
> > >  		sgs->nr_numa_running += rq->nr_numa_running;
> 
> > With this change, we'll be returning the overload indicator 
> > that we don't use for non-root domains, which will be
> > extra work in sg_lb_stats as it loops through each rq checking
> > the nr_running to update the indicator.  I was hoping to avoid
> > that if possible.
> 
> What extra work? We already load nr_running and overloaded is on-stack
> and should be quite dirty already due to that.

Okay then.  I'll need to modify the v3 patch with the following bits:

-               /* only need to update overload indicator for root domain */
-               if (!env->sd->parent && rq->nr_running > 1)
+               if (rq->nr_running > 1)
                        *overload = true;

Just sent v3 patch out before your email reached me.

Tim

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