Subject: Re: [PATCH v2] sched: Fast idling of CPU when system is partially
 loaded
From: Tim Chen <tim.c.chen@linux.intel.com>
To: Jason Low <jason.low2@hp.com>
Cc: Andi Kleen <andi@firstfloor.org>, Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@elte.hu>, Michel Lespinasse <walken@google.com>,
        Rik van Riel <riel@redhat.com>,
        Peter Hurley <peter@hurleysoftware.com>,
        Davidlohr Bueson <davidlohr@hp.com>, linux-kernel@vger.kernel.org
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Date: Mon, 23 Jun 2014 10:01:02 -0700
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On Mon, 2014-06-23 at 09:44 -0700, Jason Low wrote:
> On Mon, 2014-06-23 at 18:22 +0200, Andi Kleen wrote:
> > > So I don't get why we can't do the below; I think Jason tried to ask the
> > > same...
> > 
> > The important part for performance is to minimize the cache line transfers. Your
> > unconditional variant would cause more dirty cache lines than Tim's,
> > right?
> 
> How about that change to the update_sg_lb_stats() call site along with:
> 
>                 sgs->group_load += load;
>                 sgs->sum_nr_running += rq->nr_running;
> -               if (overload && rq->nr_running > 1)
> +               if (!env->sd->parent && rq->nr_running > 1)
>                         *overload = true;
> 
> which keeps it as a conditional to avoid unnecessarily setting overload
> when it's not used, and still get those benefits Peter mentioned.

I think this change will satisfy both needs.  I'll re-spin a v3 patch
with this modification if there're no objections.

Tim

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