linux-kernel - Re: [PATCH] sched: Reduce the rate of needless idle load balancing

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Message-ID: <1400619878.2970.276.camel@schen9-DESK>
Date:	Tue, 20 May 2014 14:04:38 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Jason Low <jason.low2@...com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <len.brown@...el.com>, Russ Anderson <rja@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Hedi Berriche <hedi@....com>, Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Reduce the rate of needless idle load balancing

On Tue, 2014-05-20 at 13:59 -0700, Tim Chen wrote:
> On Tue, 2014-05-20 at 13:51 -0700, Jason Low wrote:
> > On Tue, May 20, 2014 at 1:17 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> > 
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 9b4c4f3..97132db 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6764,12 +6764,17 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> > >
> > >                 rq = cpu_rq(balance_cpu);
> > >
> > > -               raw_spin_lock_irq(&rq->lock);
> > > -               update_rq_clock(rq);
> > > -               update_idle_cpu_load(rq);
> > > -               raw_spin_unlock_irq(&rq->lock);
> > > -
> > > -               rebalance_domains(rq, CPU_IDLE);
> > > +               /*
> > > +                * If time for next balance is due,
> > > +                * do the balance.
> > > +                */
> > > +               if (time_after(jiffies + 1, rq->next_balance)) {
> > 
> > Hi Tim,
> > 
> > If we want to do idle load balancing only when it is due for a
> > balance, shouldn't the above just be "if (time_after(jiffies,
> > rq->next_balance))"?
> 
> If rq->next_balance and jiffies are equal, then
> time_after(jiffies, rq->next_balance) check will be false and
> you will not do balance.  But actually you want to balance
> for this case so the jiffies+1 was used.

So maybe I should switch the check to 
if (time_before(rq->next_balance, jiffies))

Tim


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