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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: Davidlohr Bueso <davidlohr@hp.com>
Cc: Joe Perches <joe@perches.com>, Jason Low <jason.low2@hp.com>,
        Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
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Date: Wed, 21 May 2014 11:49:59 -0700
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On Wed, 2014-05-21 at 11:26 -0700, Davidlohr Bueso wrote:
> On Wed, 2014-05-21 at 09:37 -0700, Tim Chen wrote:
> > On Tue, 2014-05-20 at 18:15 -0700, Joe Perches wrote:
> > > On Tue, 2014-05-20 at 14:04 -0700, Tim Chen wrote:
> > > > 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@linux.intel.com> wrote:
> > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > []
> > > > > > 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))
> > > 
> > > time_after_eq() or time_is_after_eq_jiffies()
> > > 
> > > 
> > 
> > I prefer time_after_eq to keep the code style consistent with the
> > rest of the code in fair.c.
> 
> Should all the code be updated then? We should use the existing
> interfaces if available.
> 

BTW, if this code was to be updated, time_is_before_eq_jiffies(rq->next_balance) 
check will be the correct thing to do for the patch. This expands to
time_after_eq(jiffies, rq->next_balance), which is what we want.

So something like:

                if (time_is_before_eq_jiffies(rq->next_balance)) {
                        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);
                }


But I don't think this change is making the code logic any clearer.  
I prefer time_after_eq(jiffies, rq->next_balance), which is more
readable.

Tim