linux-kernel - Re: [PATCH] sched/fair: avoid !CONFIG_SMP cpufreq update callbacks when go idle

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Date:	Mon, 27 Jun 2016 16:43:21 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<mingo@...hat.com>, <rafael.j.wysocki@...el.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: avoid !CONFIG_SMP cpufreq update callbacks
 when go idle

Dear Peter,

On Mon, 27 Jun 2016 10:37:21 +0200
Peter Zijlstra wrote:

> On Mon, Jun 27, 2016 at 03:56:03PM +0800, Jisheng Zhang wrote:
> > This is to avoid cpufreq update callbacks when we go idle on UP
> > platforms.  
> 
> How so? the patch only avoids calling the update for (!root) cgroups --
> which makes sense, but I'm not seeing how that's related to going idle,
> or UP at all.

oops, I may mis-understand below comment at cfs_rq_util_change()

                 * It will not get called when we go idle, because the idle
                 * thread is a different class (!fair),

I mis-read the comment as:

it's the "&rq->cfs == cfs_rq" check to prevent the calling when go idle.

So the patch still make sense, but I need to update the commit msg?

Thanks,
Jisheng

> 
> > 
> > Signed-off-by: Jisheng Zhang <jszhang@...vell.com>
> > ---
> >  kernel/sched/fair.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bdcbeea..236ec5b 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3121,7 +3121,8 @@ static inline void update_load_avg(struct sched_entity *se, int not_used)
> >  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> >  	struct rq *rq = rq_of(cfs_rq);
> >  
> > -	cpufreq_trigger_update(rq_clock(rq));
> > +	if (&rq->cfs == cfs_rq)
> > +		cpufreq_trigger_update(rq_clock(rq));
> >  }
> >  
> >  static inline void
> > -- 
> > 2.8.1
> >