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

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Message-ID: <1400698199.2970.303.camel@schen9-DESK>
Date:	Wed, 21 May 2014 11:49:59 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Joe Perches <joe@...ches.com>, Jason Low <jason.low2@...com>,
	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 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@...ux.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

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