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Subject: Re: [PATCH] sched: Reduce the rate of needless idle load balancing
From: Tim Chen <tim.c.chen@linux.intel.com>
To: Jason Low <jason.low2@hp.com>
Cc: Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Dimitri Sivanich <sivanich@sgi.com>, Hedi Berriche <hedi@sgi.com>,
        Andi Kleen <andi@firstfloor.org>,
        Michel Lespinasse <walken@google.com>, Rik van Riel <riel@redhat.com>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Date: Tue, 20 May 2014 14:12:38 -0700
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On Tue, 2014-05-20 at 14:09 -0700, Jason Low wrote:
> On Tue, May 20, 2014 at 1:59 PM, Tim Chen <tim.c.chen@linux.intel.com> 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@linux.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.
> 
> Hi Tim, Rik
> 
> Yes, that makes sense that we want to balance if they are equal. We
> may also consider using "if (time_after_eq(jiffies,
> rq->next_balance)".

That sounds good.  Thanks.
> 
> Reviewed-by: Jason Low <jason.low2@hp.com>


Tim