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

[Tim Chen <tim.c.chen@linux.intel.com>]

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.

Tim