linux-kernel - Re: [RFC] sched: nohz_idle

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Date:	Thu, 13 Sep 2012 12:29:27 +0200
From:	Mike Galbraith <efault@....de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel@...r.kernel.org, linaro-dev@...ts.linaro.org,
	mingo@...hat.com
Subject: Re: [RFC] sched: nohz_idle_balance

On Thu, 2012-09-13 at 10:19 +0200, Peter Zijlstra wrote: 
> On Thu, 2012-09-13 at 08:49 +0200, Mike Galbraith wrote:
> > On Thu, 2012-09-13 at 06:11 +0200, Vincent Guittot wrote: 
> > > On tickless system, one CPU runs load balance for all idle CPUs.
> > > The cpu_load of this CPU is updated before starting the load balance
> > > of each other idle CPUs. We should instead update the cpu_load of the balance_cpu.
> > > 
> > > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > > ---
> > >  kernel/sched/fair.c |   11 ++++++-----
> > >  1 file changed, 6 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 1ca4fe4..9ae3a5b 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4794,14 +4794,15 @@ static void nohz_idle_balance(int this_cpu, enum cpu_idle_type idle)
> > >  		if (need_resched())
> > >  			break;
> > >  
> > > -		raw_spin_lock_irq(&this_rq->lock);
> > > -		update_rq_clock(this_rq);
> > > -		update_idle_cpu_load(this_rq);
> > > -		raw_spin_unlock_irq(&this_rq->lock);
> > > +		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(balance_cpu, CPU_IDLE);
> > >  
> > > -		rq = cpu_rq(balance_cpu);
> > >  		if (time_after(this_rq->next_balance, rq->next_balance))
> > >  			this_rq->next_balance = rq->next_balance;
> > >  	}
> > 
> > Ew, banging locks and updating clocks to what good end?
> 
> Well, updating the load statistics on the cpu you're going to balance
> seems like a good end to me.. ;-) No point updating the local statistics
> N times and leaving the ones you're going to balance stale for god knows
> how long.

Sure, the goal is fine, but I wonder about the price vs payoff.  I was
thinking perhaps the redundant updates should go away instead, unless
stats are shown to be causing real world pain.

-Mike

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